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UNIVERSITY 
OF FLORIDA 
LIBRARY 




PLANT SUCCESSION AND 
INDICATORS 



A Definitive Edition of Plant 
Succession and Plant Indicators 



BY 

FREDERIC E. CLEMENTS 

Carnegie Institution of Washington 



THE H. W. WILSON COMPANY 

New Yobk City 
1928 



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Ck)PTBIGHT, 1928 
BY 

FREDERIC E. CLEMENTS 



Printed in the United States of America 




W.B.CONKEY 
COMPANY 



PREFACE 

This is a combined and condensed edition of "Plant Succession" and 
"Plant Indicators," published by the Carnegie Institution of Washington 
in 1916 and 1920 respectively, and embodying the results of researches carried 
out under its auspices. The original editions having been exhausted, the 
Carnegie Institution has granted permission to the author to undertake the 
present publication but without responsibility on the part of the Institution. 
The two books were designed to be companion volumes, the one dealing with 
the concepts and principles, the other with the applications of the develop- 
mental method. In consequence, it seems entirely appropriate to combine 
them in a single volume, with a corresponding gain in convenience and 
economy. The expense of a new edition of each book has appeared prohibitive 
when the requirements of investigators are kept in mind, and hence every 
effort has been made to keep costs at the minimum without the sacrifice of 
essentials. It has been regarded as a scientific duty to meet the growing 
demand during the years the books have been out of print, as well as to 
render them available in supplying the background for the books mentioned 
below. 

It has been necessary to disregard the large amount of new material as 
well as to omit considerable portions of the text in order to bring the two 
books within the compass of one volume. The comprehensive nature of the 
treatments makes it possible to do this without serious harm to the main 
themes, and especially since the portions omitted are to be expanded into 
as many collateral books with a full account of the researches since 1914. 
Thus, chapters X and XI, dealing with the studies of succession in North 
America and Eurasia, and chapters XII, XIII, and XIV, treating of the 
new field of paleo-ecology, have been omitted from ' ' Plant Succession, ' ' while 
chapter IV, containing an account of the climax formations of western North 
America, has been left out of "Plant Indicators." Materially expanded in 
scope and detail, these respective portions furnish the themes for as many 
books, now well advanced in preparation. The chapters that constitute the 
main body of the two treatises have been reprinted essentially intact. The 
plates and text figures have necessarily been reduced by the omission of the 
several chapters, and reasons of economy have led to a further reduction. 

Closely correlated with this series is the second volume of ' ' Climatic Cycles 
and Tree Growth ' ' by Douglass, which has just come from the press, and two 
related books, one devoted to a consideration of rainfall cycles, the other to 
an account of the cyclic changes of climate and vegetation since the Pleistocene. 
During the decade just passed the functions of the community have been 
treated in outline in "Experimental Vegetation," and this study has been 
carried much further in "Plant Competition," which is on the eve of publica- 
tion. With these are intimately associated the four books on root develop- 






ment and behavior by Weaver and his colleagues (1919, 1920, 1922, 1924), 
while the factors of the habitat have been the theme of "Aeration and 
Air-Content" (1921) and "The Phytometer Method" (1924). All these 
and the projects related to them have been summarized each year in the 
Year Book of the Carnegie Institution and the annual report of Ecological 
Research, reprinted from it. 

Frederic E. Clements 
Mission Canton, 
Santa Barbara 
October 30, 1927 



CONTENTS 



PAGE 

Preface iii 

List of illustrations xiii 

I. Concept and Causes of Succession. 

The formation an organism 3 

Universal occurrence of succession 3 

Viewpoints of succession 3 

Succession and sere 4 

Sere and cosere 4 

Processes in succession 4 

Causes of Succession. 

Relation of causes 5 

Kinds of causes 5 

Proximate and remote causes 5 

Essential Nature of Succession. 

Developmental aspect 6 

Functional aspect 7 

II. Genekal Historical Summary. 

Early Investigations. 

King, 1685 8 

Degner, 1729 9 

Button, 1742 9 

Biberg, 1749 10 

Anderson, 1794 10 

De Luc, 1806 10 

Rennie, 1810 12 

Dureau, 1825 13 

Steenstrup, 1842 14 

Reissek, 1856 16 

Vaupell, 1857 17 

von Post, 1861 17 

Gremblich, 1876 18 

Miiller, 1878-1887 18 

Other investigations 19 

Recent Investigations. 

Blytt, 1876 21 

Hult, 1885-1887 22 

Warming, 1891 23 

MacMillan, 1894-1896 23 

Warming, 1895 25 

Graebner, 1895 25 

Pound and Clements, 1898-1900 25 

Schimper, 1898 26 

Cowles, 1899 27 

Cowles, 1901 28 

Clements, 1902-1904 28 

Clements, 1904 29 

Frtih and Schroter, 1904 29 

Clements, 1905-1907 30 

Moss, 1907-1910 30 

Clements, 1910 30 

Cowles, 1911 31 

Shantz, 1911 31 

Tansley, 1911 31 

MacDougal, 1914 31 



PAOE 



III. Initial Causes. 



Significance of bare areas 33 

Modifications of development 33 

Processes as causes 34 

Change of conditions 34 

Fundamental nature of water-content. ... 34 

Kinds of initial causes 35 

Physiography 36 

Topographic Causes. 

Topographic processes 36 

Kinds of processes 37 

Base-leveling 38 

Erosion. 

Nature 39 

Agents of erosion 39 

Rate and degree of erosion 40 

Fragmentary and superficial erosion 40 

Bare areas due to water erosion 41 

Bare areas due to wind erosion 41 

Bare areas due to gravity 41 

Bare areas due to ice action 41 

Deposit. 

Significance 42 

Agents of deposit 42 

Manner of deposit 43 

Rate and depth of deposit 43 

Place of deposit 44 

Distance of transport 44 

Fragmentary and local deposit 45 

Sterility of deposits 45 

Bare areas due to deposit by moving water 45 

Bare areas due to waves and tides 46 

Composition and water-content of alluvial 

deposits 46 

Bare areas due to deposit by ground waters 46 

Bare areas due to deposit by wind 47 

Deposit by ice and snow 47 

Bare areas due to deposit by glaciers 47 

Bare areas due to deposit by ice and snow. 48 

Bare areas due to deposit by gravity 4S 

Bare areas due to volcanic deposits 49 

Ponding and draining 49 

Kinds of lakes and ponds 50 

Life-history of a lake 50 

Drainage 51 

Elevation and Subsidence. 

Elevation and subsidence 52 

New areas due to elevation 52 

Subsidence 53 

Earthquakes 54 

Similarity of topographic processes 54 

Edaphiv Causes. 

Nature 55 



vli 



Vlll 



CONTENTS 



PAGE 

III. Initial Causes — Continued. 
Climatic Causes. 

Role 55 

Bare areas due to climatic factors directly. 55 

Bare areas due to drouth 56 

Bare areas due to wind 56 

Bare areas due to snow, hail, and frost. ... 56 

Bare areas due to lightning 57 

Bare areas due indirectly to climatic factors 57 

Sudden changes of climate 57 

Biotic Causes. 

General relations 58 

Action and effect 58 

Bare areas due to destruction of vegeta- 
tion alone 59 

Bare areas with dry or drier soils 59 

Bare areas with wet soils or water 60 

Primary and Secondary Areas. 

Distinction 60 

Sterility of primary and secondary areas. 61 

Denudation 61 

Methods of denudation 61 

Depth of removal or deposit 62 

Rate and extent of removal 62 

IV. Ecesic Causes. 

Nature 63 

Aggregation. 

Concept and r61e 63 

Effects of simple aggregation 63 

Relation to denuded areas 64 

Interaction of aggregation and migration. 64 

Migration. 

Concept 64 

Mobility 64 

Seed-production 65 

Influence of the organ used 65 

Influence of the migration contrivance. ... 66 

Role of migration agents 67 

Destructive action of agents 67 

Direction of migration 67 

Ecesis. 

Nature and r61e 68 

Germination 69 

Fate of seedling 70 

Growth 71 

Reproduction 71 

Ecesis in bare areas 71 

Competition. 

Nature 72 

Competition and dominance 72 

Competition in air and in soil 73 

Role of competition in succession 73 

Invasion. 

Nature and role 75 

Kinds of invasion 75 

Manner of invasion 76 

Barriers 77 

Biological barriers 77 

Changes in barriers 78 



PAGE 

V. Reactions 

Concept and nature 79 

Role in succession 80 

Previous analyses of reaction 80 

Kinds of reactions 81 

Soil Formation. 

Manner 81 

Reaction by accumulating plant bodies or 

parts 81 

Reaction by accumulating plant concre- 
tions 83 

Reaction by producing weathering 83 

Reaction upon wind-borne material 84 

Reaction upon water-borne detritus 85 

Reaction upon slipping sand and gravel.. . 86 

Soil Structure. 

Reaction by adding humus 86 

Reaction by compacting the soil 87 

Reaction by preventing weathering or ero- 
sion 88 

Water-Content. 

Reaction by increasing water-content 88 

Reaction by decreasing water-content. .... 89 

Nutrients and Solutes. 

Reaction by adding nutrients or foodstuffs 89 

Reaction by decreasing nutrients 89 

Reaction by producing acids 90 

Reaction by producing toxins 90 

Soil Organisms. 

Reaction by means of parasites 91 

Reaction by means of saprophytes 92 

Air Reactions. 

Reaction upon light 92 

Reaction upon humidity, temperature, and 

wind 94 

Reaction upon local climate 94 

Reaction upon aerial organisms 95 

Correlation of reactions 96 

Quantitative study of reactions 96 

VI. Stabilization and Climax. 

Stabilization 98 

Causes of stabilization ; 98 

Relation to the climax 98 

Degree of stabilization 99 

Life-History Stages. 

Nature 100 

Kinds of stages 100 

Role of life forms 100 

Reasons why plants disappear 102 

Reasons why plants appear at certain 

stages 102 

Reasons why plants appear before their 

proper time 103 

Initial stages 103 

Medial stages 105 



CONTENTS 



uc 



FAQE 

VI. Stabilization and Climax — Continued. 
The Climax. 

Concept 105 

Nature 100 

Relation to succession 106 

Kinds of climaxes 107 

Subclimaxes 107 

Potential climaxes 108 

Changes of climate 109 

Preclimax and postclimax 110 

Changes of climax 110 

VII. Structure and Units of Vegetation. 
Development and Structure. 

Relation 112 

Kinds of structure 112 

Zonation 112 

Relations of climax zones 114 

Significance of alternation 116 

Developmental relation of layers 116 

Relation of seasonal aspects 117 

The Units of Vegetation. 
Historical Summary : 

The formation concept 117 

Grisebach's concept of the formation. 117 

Drude'g concept 118 

Clements's concept 119 

Moss's concept 119 

Schroter's concept 120 

Gradmann's concept 120 

Warming's concept 121 

Negri's concept 123 

Correlation of divergent views 123 

Significance of development 124 

Earlier suggestions of developmental 

view 124 

The Formation : 

Developmental concept of the forma- 
tion 125 

Analysis of the formation 126 

Formation units 126 

Formation 127 

Names of formations 128 

Climax Units : 

Association 129 

Consociation 130 

Society 131 

Bases 132 

Kinds of societies 133 

Aspect societies 133 

Layer societies 134 

Cryptogamic societies 134 

Terminology 134 

Changes of rank or dominance 135 

Clan 135 

Serai Units : 

Nature and significance 136 

Associes 137 

Consocies 138 

Socies 139 

Colony 139 

Family 140 

Summary of units 140 

Mixed communities 140 

Nomenclature of units 141 

Formation groups 143 

Bases 144 

Developmental groups 144 



PAOE 

VIII. Direction of Development. 

Development always progressive 147 

Nature of regression 147 

Course of development 148 

Regression and retrogression 148 

Nilsson's view 148 

Cowles's view 149 

Cajander's view 151 

Sernander's view 151 

Moss's view 152 

Hole's view 156 

Conversion of forest 157 

Status of forest in Britain 158 

Artificial conversion 159 

Graebner's studies : Conversion of forest 

to heath 159 

Conversion of forest into moor 161 

Causes of conversion 163 

Possibility of backward development 164 

Degeneration 165 

Regeneration 165 

Correlation of progressive developments. . 166 

Convergence 167 

Normal movement 168 

Divergence 169 

IX. Classification of Seres. 

Historical 170 

Clements's system 170 

Normal and anomalous succession 171 

Primary and secondary succession 171 

Cowles's system 173 

Possible bases of classification 177 

Developmental basis of classification 177 

Initial areas and causes 178 

Relative importance of bases 179 

The climax as a basis 179 

Recognition of climax areas 179 

Climaxes of North American vegetation. 181-182 

Subclimaxes 183 

Extent and relationship of climaxes 183 

Names of climaxes 183 

Priseres and subseres 184 

Hydroseres and xeroseres 184 

Phylogenetic system 185 

X. The Investigation of Succession. 

Primary methods 186 

Special methods 187 

The Quadrat Method. 

Concept and significance 187 

Kinds of quadrats 188 

List quadrat 188 

Chart quadrat 189 

Permanent quadrat 190 

Denuded quadrat 192 

Quadrat series and sequences 192 

Various quadrats 193 

The transect 198 

The bisect 195 

The migration circle 196 

Methods of Mapping. 

Methods 19S 

Community charts and ecotone maps 198 

Survey maps 199 

Climax maps 199 



CONTENTS 



PAGE 

X. The Investigation of Succession — Con- 
tinued. 
Instrumental Methods. 

General considerations 200 

Measurement of reactions 201 

Measurement of water reactions 201 

Measurement of light reactions 202 

Growth Methods. 

Ring counts 203 

Burn-scars 204 

XI. Concept and History. 

The practical aspect 209 

The scientific aspect 209 

historical. 
Agricultural Indicators. 

Hilgard, 1860 211 

Chamberlin, 1877 211 

Merriam, 1898 212 

Hilgard, 1906 214 

Clements, 1910 215 

Shantz, 1911 216 

Kearney, Briggs, Shantz, McLane, and 

Piemeisel, 1914 217 

Shantz and Piemeisel, 1917 218 

Shantz and Aldous, 1917 219 

Weaver, 1919 219 

Forest Indicators. 

Cajander, 1909 220 

Clements, 1910 220 

Pearson, 1913-1914 221 

Zon, 1915 222 

Hole and Singh, 1916 222 

Korstian, 1917 223 

Grazing Indicators. 

Smith, 1899 225 

Bentley, 1902 226 

Griffiths, 1901, 1904, 1907, 1910, 1915. . . 227 

Sampson, 1908, 1909, 1913, 1914 228 

Jardine, 1908, 1909, 1910, 1913 229 

Wooton, 1915, 1916 229 

Jardine and Hurtt, 1917 230 

Jardine and Anderson, 1919 231 

Sarvis, 1919 231 

Ghresard and Water Requirement Studies. 

Significance 232 

The chresard 232 

Gain, 1895 233 

Kihlmann, 1890 233 

Briggs and Shantz, 1912 233 

Water requirement 234 

concept. 

General 234 

Animals as indicators 235 

Plant and community 235 

Sequences 236 

Direct and indirect sequences 237 

Direction of indication 238 

Scope 238 

Materials 239 

Basing studies 240 



page 
XII. Bases and Criteria. 

BASES AND METHODS OF DETERMINATION. 

Fundamental relations 241 

The Physical Basis. 

Direct and indirect factors 242 

Controlling and limiting factors 242 

Climatic and edaphic factors 243 

Climates and habitats 244 

Variation of climate and habitat 245 

Inversion of factors 247 

Measurement of habitats 248 

The Physiological Basis. 

Kinds of response 249 

Effect of habit 249 

Individuality in response 250 

Effect of extreme conditions 250 

Phytometers 251 

The Associational Basis. 

Nature of association 253 

Dominants 253 

Equivalence of dominants 254 

Absence of dominants 256 

Subdominants 256 

Secondary species 257 

Plant and animal association 257 

The Successional Basis. 

Scope 258 

Sequence of indicators 258 

Major successions as indicators 259 

The Experimental Basis. 

Nature 260 

Essentials 261 

INDICATOR CRITERIA. 

Nature and kinds of criteria 261 

Species and genera 262 

Life-Forms. 

History 263 

Pound and Clements, 1898-1900 264 

Raunkiaer, 1905 264 

Warming, 1908 266 

Drude, 1913 267 

Comparison of the systems 268 

Vegetation-forms 269 

Indicator significance of vegetation-forms. 270 

Habitat-Forms. 

Concept and history 271 

Warming's system 271 

Modifications of Warming's system 272 

Indicator value 273 

Ecads 274 



CONTENTS 



XI 



PAOE 

XII. Bases and Criteria — Continued. 

indicator criteria — continued. 

Qrowth-Forms. 

Nature 275 

Kinds 276 

Indicator relations 276 

Standard plants for growth correlations. . 277 

Competition-forms 278 

Communities as Indicators. 

Value 279 

Kinds of communities 279 

Community structures 280 

Alternes 280 

Layers 281 

Aspects 282 

XIII. Kinds of Indicators. 
Basis of distinction 283 

FACTOR INDICATORS. 

Basis and kinds 283 

Quantitative sequences 284 

Climatic and edaphic indicators 284 

Water indicators 285 

Light indicators 286 

Temperature indicators 288 

Indicators of solutes 290 

Saline indicators 290 

Lime indicators 291 

Aeration indicators 292 

Indicators of factor-complexes 295 

Soil indicators 295 

Slope-exposure indicators 295 

Altitude indicators 296 

Organism indicators 297 

PROCESS INDICATORS. 

Nature 298 

Kinds 299 

Fire indicators 299 

Lumbering indicators 300 

Cultivation indicators 301 

Grazing indicators 302 

Indicators of irrigation and drainage 302 

Construction indicators 303 

Physiographic indicators 304 

Climatic indicators 305 

PRACTICE INDICATORS. 

Nature 305 

Kinds 306 

PALEIC INDICATORS. 

Paleo-ecology 306 

Nature of paleie indicators 307 

Kinds 308 

Paleie indicators of climates and cycles. . . 310 

Paleie indicators of succession 310 

Plant indicators of animals 311 

Animal indicators of plants 311 

XIV. Agricultural Indicators. 

General relations 313 

land classification. 

Nature 313 

Relation to practices 314 

Proposed bases of classification 314 



PAGE 

XIV. Agricultural Indicators — Continued. 

climatic cycles. 
The indicator method of land classification 316 

Use of climax indicators 316 

Soil indicators 317 

Shantz's results 318 

A SYSTEM OF LAND CLASSIFICATION. 

Bases 321 

Classification and use 321 

Methods 322 

CLIMATIC CYCLES. 

Nature 323 

The 11-year cycle 323 

Evidences 324 

Periods of drought 326 

Recurrence of drought periods 327 

Significance of the sun-spot cycle 328 

Prediction of drought periods 329 

Utilization of cycles 330 

FARMING INDICATORS. 

Types of farming 331 

Relation of types of farming to indicators 331 
Edaphic indicators of types of farming. . 332 

CROP INDICATORS. 

Nature and kinds 333 

Climatic indicators of the types of crops. 334 

Climatic indicators of kinds of crops. . . . 335 

Climatic indicators of varieties 335 

Life zones and crop zones 336 

Edaphic indicators of crops and methods. . 337 

Indicators of native or ruderal forage crops 338 

AGRICULTURAL PRACTICE AND CLIMATIC 
CYCLES. 

Cycles of production 33S 

The excess-deficit balance 342 

Anticipation of cycles 344 

XV. Grazing Indicators. 
Kinds of grazing 346 

GRAZING TYPES. 

Kinds of grazing indicators 347 

Significance of climax types 348 

Formations as indicators 349 

Associations as indicators 349 

Consociations as indicators 350 

Local grazing types 351 

Savannah as an indicator 352 

Kinds of savannah 354 

Savannah in relation to fire and grazing. . 355 

Significance of serai types 356 

Prisere communities as indicators 356 

Subsere communities as indicators 358 

Fire indicators and grazing 359 

CARRYING CAPACITY. 

Nature and significance 360 

Determining factors 360 

Relation to communities and dominants.. 361 

Nutrition content 362 

Relation to climatic cycles 36S 

Relation to rodents 869 

Relation to herd and management 369 

Measurement of carrying capacity ;?T0 

Present and potential carrying capacity.. 371 



Xll 



CONTENTS 



PAGE 

XV. Grazing Indicators — Continued. 

OVERGRAZING. 

Nature 372 

Causes 373 

Indicators of overgrazing 373 

Societies as indicators 375 

Halfshrubs as indicators 375 

Cacti as indicators 376 

Shrubs as indicators 377 

Annuals as indicators 377 

Prairie and plains indicators 378 

Desert plains indicators 378 

Bunch-grass prairie indicators 379 

Great Basin indicators 380 

Overgrazing in the past 381 

Succession and cycles 383 

Relation of tall-grasses and short-grasses. 384 

Overgrazing cycles 385 

RANGE IMPROVEMENT. 

History 386 

Prerequisites 387 

Essential factors 388 

Proper stocking 388 

Rotation grazing 390 

Rodent eradication 392 

Eradication of poisonous plants 393 

Eradication of weeds and cacti 396 

Eradication of brush 396 

Manipulation of the range 397 

Plant introduction on the range 398 

Prerequisites for seeding and planting. . . . 401 

New investigations 402 

Forage development 403 

Water development 404 

Herd management 405 

ESSENTIALS OF A GRAZING POLICY. 

A proper land system 406 



PAGE 

XV. Grazing Indicators — Continued. 
essentials of a grazing polict — continued. 

Essentials 407 

Classification and range surveys 407 

Production cycles 408 

Range management surveys 409 

XVI. Forest Indicators. 

Nature 410 

Kinds of indicators 410 

forest types. 

Bases 411 

Comparison of views 416 

Forest sites 417 

Succession as a basis 41§ 

Significance 419 

climatic and edaphic indicators. 

Climatic indicators 419 

Edaphic indicators 422 

Water-content indicators 422 

Light indicators 423 

Site indicators 423 

Growth as an indicator 424 

Burn indicators 427 

Grazing indicators 429 

Cycle indicators 431 

PLANTING INDICATORS. 

Kinds 431 

Prerequisites for planting and sowing. . . . 432 

Use of climatic cycles 433 

Reforestation indicators 433 

Afforestation indicators 436 

Bibliography 438 



LIST OF ILLUSTRATIONS. 
PLATES. 



PAGE. 

Plate 1. 

A. Stages of a sandhill sere as seen 

in three successive blowouts, 
Halsey, Nebraska 4 

B. Section of a peat deposit, serving 

as a record of the cosere, 
"Burton Lake," Lancashire, 

England 4 

Plate 2. 

A. Primary bare area, due to weather- 

ing, Mount Garfield, Pike's Peak, 
Colorado 34 

B. Secondary bare area, due to wind 

erosion, Morainal Valley, Pike's 

Peak, Colorado 34 

Plate 3. 

A. Superficial erosion by water on 

clay hills, La Jolla, California. 42 

B. Bare areas due to the action of 

gravity, Cafion of the Yellow- 
stone River, Yellowstone Park. 42 

C. Bare areas due to the action of ice, 

Yosemite Valley, California... 42 
Plate 4. 

A. Local and fragmentary deposit in 

a young ravine, bad lands, 
Scott's Bluff, Nebraska 46 

B. Sand-bars due to deposit in 

streams, North Platte River, 
Scott's Bluff, Nebraska 46 

C. Silting up of the Soledad Estuary, 

La Jolla, California 46 

Plate 5. 

A. Terminal moraine of the Nis- 

qually Glacier, Mount Rainier, 
Washington ; bare area due to 
deposit by a glacier 48 

B. Talus slopes of Scott's Bluff. 

Nebraska ; bare areas due to 

gravity 48 

Plate 6. 

A. Primary area colonized by mosses, 

terminal moraine of the Illecille- 
waet Glacier, Glacier, British 
Columbia 60 

B. Secondary area colonized by Sal- 

sola, on a railway embank- 
ment, bad lands, Scott's Bluff, 

Nebraska 60 

Plate 7. 

A. Superficial wind erosion, Dune 

Point, La Jolla, California.... 62 

B. Deep-seated water erosion, Torrey 

Pines, Del Mar, California. ... 62 
Plate 8. 

A. Family of Pachylophus caespitosus on 

gravel-slide, Alpine Laboratory, 
Colorado 66 

B. Colony of Suaeda and Atriplew in 

a depression, bed of a former 
salt lake, Hazen, Nevada 66 



paoe. 
Plate 8 — Continued. 
C. Tumbleweed, Salsola, on the Great 

Plains, Akron, Colorado 66 

Plate 9. 

A. Ecesis in a primary area, sum- 

mit of Pike's Peak, Colorado. . 72 

B. Ecesis in a secondary area de- 

nuded by hot water, Norris 
Geyser Basin, Yellowstone 

Park 72 

Plate 10. 

A. Reaction by the accumulation of 

plant remains in water ; peat 
beds, "Burton Lake," Lan- 
cashire, England 84 

B. Reaction by causing weathering, 

Pilot Knob, Pike's Peak, Colo- 
rado 84 

Plate 11. 

A. Reaction by preventing weather- 

ing, crustose lichens, Picture 
Rocks, Tucson, Arizona 88 

B. Consocies of Chrysothamnus reduc- 

ing water erosion in marginal 
gullies of bad lands, Scott's 

Bluff, Nebraska 88 

Plate 12. 

A. Initial stages of a xerosere, lichens, 

mosses and liverworts, Picture 
Rocks, Tucson, Arizona 104 

B. Initial stage of a hydrosere, 

Nijmphaea polysepala, Two 
Ocean Lake, Yellowstone Park. 104 
Plate 13. 

A. Climax prairie of Stipa and Agro- 

pyrum, Winner, South Dakota. 106 

B. Climax forest of Pseudotsuga, 

Tsuga and Thuja, Mount Rain- 
ier, Washington 106 

Plate 14. 

A. Postclimaxes of scrub (Shep- 

herdia, Amelanchier, etc.) and 
of woodland (Ulmus, Fraxinus, 
Quercus) in prairie climax. 
Gasman Coulee, Minor, North 
Dakota 110 

B. Sagebrush preclimax (Artemisia 

tridentata) and Pinus ponde- 
rosa climax, Estes Park, Colo- 
rado 110 

Plate 15. 

A. Montane forest association, Pinus- 

Abies-hylium (P. ponderosa, P. 
lambertiana, Abies concolor) , 
Yosemite Park, California 130 

B. Yellow-pine consociation, Pinctum 

ponderosae, Prospect, Oregon.. 130 
Plate 16. 

A. Lupine society, Lupinilc plattensia, 
in mixed prairie. Hat Creek 
Basin, Nebraska 134 



XIV 



LIST OF ILLUSTRATIONS 



PAGE. 

Plate 16 — Continued. 
B. Clan of Pirola elliptica in forest, 

Lake Calhoun, Minnesota 134 

Plate 17. 

A. Grass associes of Andropogon 

scoparius and Galamovilfa longi- 
folia, Crawford, Nebraska 138 

B. Pentstemon socies (P. seoundi- 

florus) in gravel, Manitou, Colo- 
rado 138 

Plate 18. 

A. Artemisia-Populus-ecotone, Fallon, 

Nevada 140 

B. Picea-Populus-mictium, Alpine 

Laboratory, Colorado 140 

Plate 19. 

A. Denudation in moorland, the peat- 

Jiags capped here and there with 
bilberry (Vaccinium myrtillus) ; 
"retrogression" of the cotton- 
grass moor (Eriophoretum) . . . 152 

B. Degeneration of beechwood due to 

rabbits, Holt Down, Hampshire, 

England 152 

Plate 20. 

A. Destruction of woodland of Pinus 

torreyana by fire and erosion 
and replacement by chaparral, 
Del Mar, California 164 

B. Root-sprouting from the base of 

burned chaparral dominants, 
(Quercus, Arctostaphylus, etc.), 
Mount Tamalpais, California. . 164 

C. Destruction of mixed prairie and 

invasion by woodland and scrub, 
bad lands, Crawford, Nebraska 164 
Plate 21. 

A. Deciduous forest climax of Acer- 

Fagus, Three Oaks, Michigan.. 178 

B. Subalpine forest climax of Picea- 

Abies, Mount Blanca, Colorado. 178 
Plate 22. 

A. Prisere alternes showing the serai 

stages from the bare diatom 
marsh to the lodgepole subcli- 
max, Firehole Basin, Yellow- 
stone Park 184 

B. Subsere alternes due to the re- 

moval of sods for adobe houses, 
showing three stages: (1) 
rushes (2) salt-grass (3) Ane- 
mopsis, Albuquerque, New Mex- 
ico 184 

Plate 23. 

A. Hydrosere of Batrachium, Pota- 

mogeton, Nymphaea, Carex, etc., 
Lily Lake, Estes Park, Colo- 
rado 186 

B. Xerosere of lichens, mosses, an- 

nuals and grasses on lava 
ridge, Death Valley, California. 186 
Plate 24. 

A. Short-grass (Bouteloua gracilis) 
on hard-land, Colorado Springs, 
Colorado 216 



PAGE. 

Plate 24 — Continued. 

B. Wire-grass (Aristida purpurea) in 
short-grass subclimax, Walsen- 
burg, Colorado 216 

Plate 25. 

A. Lowland mesquite (Prosopis juli- 

flora) at 2,500 feet in the San 
Pedro Valley, Arizona 246 

B. Foothill mesquite meeting oak at 

4,500 feet, Patagonia Mountains, 

Arizona 246 

Plate 26. 

A. Pentstemon gracilis as a climax 

subdominant in mixed prairie, 
Gordon, Nebraska 256 

B. Pedicularis crenulata as a serai 

subdominant in a Juncus-Carex 
swamp, Laramie, Wyoming.... 256 
Plate 27. 

A. Typha alternes, indicating pools in 

a salt-marsh, Goshen, California 286 

B. Juniperus, indicating seepage lines 

in hills of Mancos shale, Cedar, 

Colorado 286 

Plate 28. 

A. Hordeum plain and Suaeda hum- 

mocks, indicating differences in 
salt-content, Great Salt Lake, 
Utah 290 

B. Communities of Phleum-Equisetum 

and Juncus-Heleocharis mark- 
ing differences in water-content 
and aeration, Sapinero, Colorado 290 
Plate 29. 

A. Aspen, indicating an early fire, and 

sagebrush alternes a recent one, 
Strawberry Canon, Utah 300 

B. Artemisia frigida, indicating an 

old fallow field, Warbonnet 
Cation, Pine Ridge, Nebraska. . 300 
Plate 30. 

A. Artemisia filifolia indicating sandy 

soil, Canadian River, Texas... 318 

B. Bouteloua and Bulbilis on hard- 

land, Good well, Oklahoma 318 

C. Atriplex nuttallii, indicating non- 

agricultural saline land, Thomp- 
son, Utah 318 

Plate 31. 

A. Tall valley sagebrush indicating a 

deep soil for irrigation farming, 
Garland, Colorado 332 

B. A legume, Lupinus plattensis, in- 

dicating a rich moist soil, 
Monroe Canon, Pine Ridge, 
Nebraska 332 

C. Relict Stipa and Balsamorhiza in 

sagebrush, indicating a bunch- 
grass climate for dry-farming, 

Hagerman, Idaho 332 

Plate 32. 

A. Mixed prairie (Stipa comata), in- 

dicating dry-farming, Scenic, 
South Dakota 336 

B. Tall-grass (Andropogon furcatus), 

indicating humid farming, Mad- 
ison, Nebraska 336 



LIST OF ILLUSTRATIONS 



PAGE. 

Plate 32 — Continued. 

0. Bunch-grass prairie (Agropyrum- 
Festuca), indicating dry-farm- 
ing with winter rainfall, The 
Dalles, Oregon 336 

Plate 33. 

A. Grass type, Andropogon-Bulbilis- 

Bouteloua, Smoky Hill River, 
Hays, Kansas 348 

B. Weed type, Erigeron, Geranium, 

etc. in aspen forest, Pike's 
Peak, Colorado 348 

C. Browse type, Artemisia tridcn- 

tata, Beulah, Oregon 348 

Plate 34. 

A. Elymus and Agropyrum reappear- 

ing as a result of fire in sage- 
brush, Boise, Idaho 354 

B. Sagebrush dying out as a result of 

competition with Agropyrum, 

Craig, Colorado 354 

Plate 35. 

A. Mixed prairie of tall-grass {Agro- 

pyrum) and short-grass (Bul- 
bilis), Winner, South Dakota.. 302 

B. Pure turf of short-grass (Bul- 

bilis), Ardmore, South Dakota. 362 
Plate 36. 

A. Aristida-Bouteloua association in 

1917, Santa Rita Reserve, Tuc- 
son, Arizona 370 

B. The same area in 1918 after serious 

drought and overgrazing by cat- 
tle and rodents 370 

Plate 37. 

A. Aristida purpurea and divaricata, 

indicating moderate overgrazing 
on Bulbilis plains, Texhoma, 
Oklahoma 374 

B. An annual, Lepidium alyssoides, 

indicating complete overgrazing 
in a pasture, Fountain, Colo- 
rado 374 

Plate 38. 

A. Opuntia polyacantha, indicating 

serious overgrazing in mixed 
prairie, Hat Creek Basin, 
Nebraska 380 

B. Trackway relict of Stipa setigera, 

indicating the original climax 
prairie of California, now al- 
most completely destroyed by 
overgrazing, Fresno, California 380 
Plate 39. 

A. Mixed prairie of Andropogon- 
Boutcloua racemosa and Bulbi- 



page. 
Plate 39 — Continued. 

lis-Boutcloua gracilis under pro- 
tection, Wilson, Kansas 384 

B. The same prairie in an adjoining 
pasture, showing its change to a 
pure short-grass sod, Wilson, 

Kansas 384 

Plate 40. 

A. Isolation transect in Htipa-Bou- 

teloua pasture, Mandan, North 
Dakota 390 

B. Grazing exclosure with cattle- 

proof and rodent-proof units ; 
crop of winter annuals, chiefly 
Eschscholtzia mexicana ; Santa 
Rita Reserve, Tucson, Ari- 
zona 390 

Plate 41. 

A. Chamaebatia foliolosa, indicating 

fire in pine forest, Yosemite 
National Park, California 428 

B. Pteris and Rubus, indicating a re- 

cent burn, following one marked 
by Arbutus-Prunus, associes, 
Pseudotsuga forest, Eugene, Ore- 
gon 428 

Plate 42. 

A. Pine reproduction in an exclosure, 

Fort Valley Experiment Sta- 
tion, Flagstaff, Arizona 430 

B. Reproduction cycles of Picea engel- 

manni, Uncompahgre Plateau, 

Colorado 430 

Plate 43. 

A. Arbutus as an indicator for re- 

forestation, Pseudotsuga forest, 
Eugene, Oregon 434 

B. Burn reproduction of Pseudotsuga 

from seed in soil, Wind River 
Experiment Station, Washing- 
ton 434 

Plate 44. 

A. Andropogon-Calamovilfa, tall-grass 

subclimax in sandhills, indicating 
high chresard and the possibil- 
ity of afforestation, Halsey, 
Nebraska .' . 436 

B. Three-year-old plantation of jack- 

pine (Pinus divaricata) in sand- 
hills, Halsey, Nebraska 436 

C. Jack-pines 10 years after trans- 

planting, Halsey, Nebraska.... 436 



XVI 



LIST OF ILLUSTRATIONS 



TEXT-FIGURES. 



PAGE. 

1. Section of Vidnesdam moor in Den- 

mark 14 

2. Section of Lillemose moor in Den- 

mark 15 

3. Schematic representation of develop- 

ment of the hydrosere 113 

4. Quadrat showing reproduction in a 

complete burn 190 

5. Quadrat showing seedlings of Pinus 

murrayana in a Vaccinium cover 191 

6. Belt transect through forest of lodge- 

pole pine, bare burn and half burn 194 

7. Bisect of sandhills mixed association 

in eastern Colorado 196 

8. Bisect of the Bulbilis-Bouteloua sub- 

climax in eastern Colorado 196 

9. Transect of a migration arc 197 

10. Zones of a fairy ring due to Agari- 

cus tabularis 218 

11. Diagram of the climax and serai 

communities of the formation.... 280 

12. The 11-year cycle during the last 

250 years, as shown by the yellow 
pine and Sequoia, 324 



PAGE. 

13. Double and triple sun-spot cycle in 

yellow pine from 1700 to 1900 

A. D 326 

14. 2-year cycle in a sequoia 339 

15. Graph of total and seasonal rainfall 

at Williston, North Dakota 340 

16. Graph of total and seasonal rainfall 

at Cheyenne, Wyoming 341 

17. Graph of total and seasonal rain- 

fall at Akron, Colorado 342 

18. Graph of total and seasonal rainfall 

at Amarillo, Texas 343 

19. Cycles of rainfall in the Ohio Valley, 

and in Illinois 345 

20. Cycles in the yield of corn and in 

the rainfall of its critical period 

of growth 345 

21. Pastures for the intensive study of 

carrying capacity and rotation 
grazing 389 

22. Isolation transect for measuring 

cyclic changes in yield 390 

23. Arrangement of corrals, sheds and 

scales, Mandan, North Dakota. . . 391 

24. Indicators of planting sites in the 

various zones 435 



PLANT SUCCESSION 

AN ANALYSIS OF THE DEVELOPMENT 
OF VEGETATION 

An Abridgment of Publication No. 242 

CARNEGIE INSTITUTION OF WASHINGTON 

1916 



I. CONCEPT AND CAUSES OF SUCCESSION. 

The formation an organism. — The developmental study of vegetation neces- 
sarily rests upon the assumption that the unit or climax formation is an 
organic entity (Research Methods, 199). As an organism the formation 
arises, grows, matures, and dies. Its response to the habitat is shown in 
processes or functions and in structures which are the record as well as the 
result of these functions. Furthermore, each climax formation is able to 
reproduce itself, repeating with essential fidelity the stages of its development. 
The life-history of a formation is a complex but definite process, comparable 
in its chief features with the life-history of an individual plant. 

Universal occurrence of succession. — Succession is the universal process of 
formation development. It has occurred again and again in the history of 
every climax formation, and must recur whenever proper conditions arise. 
No climax area lacks frequent evidence of succession, and the greater number 
present it in bewildering abundance. The evidence is most obvious in active 
physiographic areas, dunes, strands, lakes, flood-plains, bad lands, etc., and 
in areas disturbed by man. But the most stable association is never in com- 
plete equilibrium, nor is it free from disturbed areas in which secondary succes- 
sion is evident. An outcrop of rock, a projecting boulder, a change in soil or 
in exposure, an increase or decrease in the water-content or the light intensity, 
a rabbit-burrow, an ant-heap, the furrow of a plow, or the tracks worn by 
wheels, all these and many others initiate successions, often short and minute, 
but always significant. Even where the final community seems most homo- 
geneous and its factors uniform, quantitative study by quadrat and instru- 
ment reveals a swing of population and a variation in the controlling factors. 
Invisible as these are to the ordinary observer, they are often very consider- 
able, and in all cases are essentially materials for the study of succession. In 
consequence, a floristic or physiognomic study of an association, especially in 
a restricted area, can furnish no trustworthy conclusions as to the prevalence 
of succession. The latter can be determined only by investigation which is 
intensive in method and extensive in scope. 

Viewpoints of succession. — A complete understanding of succession is pos- 
sible only from the consideration of various viewpoints. Its most striking 
feature lies in the movement of populations, the waves of invasion, which rise 
and fall through the habitat from initiation to climax. These are marked by 
a corresponding progression of vegetation forms or phyads, from lichens and 
mosses to the final trees. On the physical side, the fundamental view is that 
which deals with the forces which initiate succession and the reactions which 
maintain it. This leads to the consideration of the responsive processes or 
functions which characterize the development, and the resulting structures, 
communities, zones, alternes, and layers. Finally, all of these viewpoints are 
summed up in that which regards succession as the growth or development 

3 



4 CONCEPT AND CAUSES OF SUCCESSION 

and the reproduction of a complex organism. In this larger aspect succession 
includes both the ontogeny and the phylogeny of climax formations. 

Succession and sere. — In the thorough analysis of succession it becomes 
evident that the use of the term in both a concrete and an abstract sense tends 
to inexactness and uncertainty. With the recognition of new kinds of succession 
it seems desirable to restrict the word more and more to the phenomenon 
itself and to employ a new term for concrete examples of it. In consequence, 
a word has been sought which would be significant, short, euphonic, and easy 
of combination. These advantages are combined in the word sere, from a root 
common to both Latin and Greek, and hence permitting ready composition in 
either. The root ser- shows its meaning in Latin sero, join, connect; sertum, 
wreath ; series, joining or binding together, hence sequence, course, succession, 
lineage. In Greek, it occurs in eipm, to fasten together in a row, and in 
aeipd, crrjpd, rope, band, line, lineage. Sere is essentially identical with series, 
but possesses the great advantage of being distinctive and of combining much 
more readily, as in cosere, geosere, etc. 

Sere and cosere. — A sere is a unit succession. It comprises the develop- 
ment of a formation from the appearance of the first pioneers through the final 
or climax stage. Its normal course is from nudation to stabilization. All 
concrete successions are seres, though they may differ greatly in development 
and thus make it necessary to recognize various kinds, as is shown later. On 
the other hand, a unit succession or sere may recur two or more times on the 
same spot. Classical examples of this are found in moors and dunes, and in 
forest burns. A series of unit successions results, in which the units or seres 
are identical or related in development. They consist normally of the same 
stages and terminate in the same climax, and hence typify the reproductive 
process in the formation. Such a series of unit successions, i. e., of seres, in 
the same spot constitutes an organic entity. For this, the term consere or 
cosere (cum, together, sere; consero, bind into a whole) is proposed, in recogni- 
tion of the developmental bond between the individual seres. Thus, while 
the sere is the developmental unit, and is purely ontogenetic, the cosere is the 
sum of such units throughout the whole life-history of the climax formation, 
and is hence phylogenetic in some degree. Coseres are likewise related in a 
developmental series, and thus may form larger groups, eoseres, etc., as indi- 
cated in the later discussion (plate 1, a, b). 

Processes in succession. — The development of a climax formation consists 
of several essential processes or functions. Every sere must be initiated, and 
its life-forms and species selected. It must progress from one stage to 
another, and finally must terminate in the highest stage possible under the 
climatic conditions present. Thus, succession is readily analyzed into initia- 
tion, selection, continuation, and termination. A complete analysis, however, 
resolves these into the basic processes of which all but the first are functions 
of vegetation, namely, (1) nudation, (2) migration, (3) ecesis, (4) competi- 
tion, (5) reaction, (6) stabilization. These may be successive or interacting. 
They are successive in initial stages, and they interact in most complex fash- 
ion in all later ones. In addition, there are certain cardinal points to be con- 
sidered in every case. Such are the direction of movement, the stages in- 
volved, the vegetation forms or materials, the climax, and the structural units 
which result. 



CLEMENTS 






• 

• 


a 


• •• .„ 

J 1 Wt 





^^ft**? ' 



4 . (**£-'<? 




?*■ 4S5 




A. Stages of a sandhill sere as seen in three successive blowouts, Halsey, Nebraska 
B. Section of a peat deposit, serving as a record of the eosere, "Burton Lake," 

Lancashire, England. 



; 



CAUSES OF SUCCESSION 5 

CAUSES OF SUCCESSION. 

Relation of causes. — Since succession is a series of complex processes, it 
follows that there can be no single cause for a particular sere. One cause ini- 
tiates succession by producing a bare area, another selects the population, a 
third determines the sequence of stages, and a fourth terminates the develop- 
ment. As already indicated, these four processes — initiating, selecting, con- 
tinuing, and terminating — are essential to every example of succession. As 
a consequence, it is difficult to regard any one as paramount. Furthermore, 
it is hard to determine their relative importance, though their difference in 
role is obvious. It is especially necessary to recognize that the most evident 
or striking cause may not be the most important. In fact, while the cause or 
process which produces a bare habitat is the outstanding one to the eye, in 
any concrete case, it is rather less important if anything than the others. 
While the two existing classifications of successions (Clements, 1904; Cowles, 
1911) have both used the initiating cause as a basis, it seems clear that this 
is less significant in the life-history of a climax formation than are the others. 
This matter is discussed in detail in Chapter IX. It will suffice to point out 
here that the same sere may result from several initial causes. 

Kinds of causes. — All of the causative processes of succession may best be 
distinguished as initiating or initial, continuing or ecesic, and stabilizing or 
climatic. At first thought, the latter seems not to be a cause at all but an 
effect. As is shown later, however, the character of a successional development 
depends more upon the nature of the climatic climax than upon anything else. 
The latter determines the population from beginning to end, the direction of 
development, the number and kind of stages, the reactions of the successive 
stages, etc. Initial causes are those which produce a new or denuded soil 
upon which invasion is possible. Such are the chief physiographic processes, 
deposition and erosion, biotic factors such as man and animals, and climatic 
forces in some degree. 

Ecesic causes are those which produce the essential character of vegeta- 
tional development, namely, the successive waves of invasion leading to a 
final climax. They have to do with the interaction of population and habitat, 
and are directive in the highest degree. The primary processes involved are 
invasion and reaction. The former includes three closely related processes, 
migration, competition, and ecesis. The last is final and critical, however, 
and hence is used to designate the causes which continue the development. 

Proximate and remote causes. — In dealing with the causes of development, 
and especially with initial causes, it must be borne in mind that forces in nature 
are almost inextricably interwoven. In all cases the best scientific method in 
analysis seems^to be to deal with the immediate cause first, and then to trace 
its origin just as far as it is possible or profitable. Throughout a climax for- 
mation, physiography usually produces a large or the larger number of devel- 
opmental areas. The influence of physiography in this respect is controlled 
or limited by the climate, which in its turn is determined by major physio- 
graphic features such as mountain barriers or ocean currents. These are 
subordinate as causes to the general terrestrial climates, which are the outcome 
of the astronomical relations between the sun and the earth. As a conse- 
quence, physiography may well be considered the immediate initial cause of 



6 CONCEPT AND CAUSES OF SUCCESSION 

the majority of primary successions, just as the chresard is the controlling 
cause of vegetation structure, though it is dependent on the one hand upon 
soil structure, and this upon physiography, and on the other upon the rain- 
fall, etc. 

Apart from the gain in clearness of analysis, greater emphasis upon the 
proximate cause seems warranted by the fact that it is the chresard to which 
the plant responds, and not the soil-texture or the physiography. In like 
manner, the invasion of a new area is a direct consequence of the action of the 
causative process and not of the remote forces behind it. The failure to consider 
the sequence of causes has produced confusion in the past (c/. Chapter III) 
and will make more confusion in the future as the complex relations of vegeta- 
tion and habitat come to be studied intensively. The difficulties involved 
are well illustrated by the following conclusion of Raunkiaer (1909) : 

"Every formation is before all dependent upon the temperature, and on the 
humidity originating from the precipitation; the precipitation is distributed 
in different ways in the soil according to its nature and surface, and hence 
comes the division into formations. It therefore can not be said that one 
formation is edaphic and another not; on the other hand, they may all be 
termed edaphic, dependent as they are on the humidity of the soil; but as 
the humidity is dependent upon the precipitation, it is most natural to say 
they are all climatic." 



ESSENTIAL NATURE OF SUCCESSION. 

Developmental aspect. — The essential nature of succession is indicated by 
its name. It is a series of invasions, a sequence of plant communities marked 
by the change from lower to higher life-forms. The essence of succession lies 
in the interaction of three factors, namely, habitat, life-forms, and species, in 
the progressive development of a formation. In this development, habitat 
and population act and react upon each other, alternating as cause and effect 
until a state of equilbrium is reached. The factors of the habitat are the 
causes of the responses or functions of the community, and these are the causes 
of growth and development, and hence of structure, essentially as in the indi- 
vidual. Succession must then be regarded as the development or life-history 
of the climax formation. It is the basic organic process of vegetation, which 
results in the adult or final form of this complex organism. All the stages 
which precede the climax are stages of growth. They have the same essential 
relation to the final stable structure of the organism that seedling and growing 
plant have to the adult individual. Moreover, just as the adult plant repeats 
its development, i. e., reproduces itself, whenever conditions permit, so also 
does the climax formation. The parallel may be extended much further. 
The flowering plant may repeat itself completely, may undergo primary 
reproduction from an initial embryonic cell, or the reproduction may be 
secondary or partial from a shoot. In like fashion, a climax formation may 
repeat every one of its essential stages of growth in a primary area, or it may 
reproduce itself only in its later stages, as in secondary areas. In short, the 
process of organic development is essentially alike for the individual and the 
community. The correspondence is obvious when the necessary difference 
in the complexity of the two organisms is recognized. 






ESSENTIAL NATURE OF SUCCESSION 7 

Functional aspect. — The motive force in succession, i. e., in the develop- 
ment of the formation as an organism, is to be found in the responses or func- 
tions of the group of individuals, just as the power of growth in the individual 
lies in the responses or functions of various organs. In both individual and 
community the clue to development is function, as the record of development 
is structure. Thus, succession is preeminently a process the progress of which 
is expressed in certain initial and intermediate structures or stages, but is 
finally recorded in the structure of the climax formation. The process is 
complex and often obscure, and its component functions yield only to persist- 
ent investigation and experiment. In consequence, the student of succession 
must recognize clearly that developmental stages, like the climax, are only a 
record of what has already happened. Each stage is, temporarily at least, a 
stable structure, and the actual processes can be revealed only by following 
the development of one stage into the succeeding one. In short, succession 
can be studied properly only by tracing the rise and fall of each stage, and not 
by a floristic picture of the population at the crest of each invasion. 



II. GENERAL HISTORICAL SUMMARY. 

In order to give students a general idea of the development of the subject, 
an account of all the earlier papers accessible is given here. After the work 
of Hult (1885), studies of succession became more frequent. In this recent 
period, those works have been selected which mark an advance in the prin- 
ciples or methods used in the investigation of development, or which endeavor 
to organize the field in some degree. The literature of the peat cosere is so 
vast, however, that only a few of the more comprehensive works can be 
mentioned here. This applies especially to the literature of Quaternary and 
earlier plant horizons, much of which has only an indirect bearing upon the 
problems of succession. This field has also produced a rich harvest of polemic 
writings, nearly all of which are ignored, with the exception that many of 
the titles are listed in the bibliography. 

EARLY INVESTIGATIONS. 

King, 1685. — "While there is abundant evidence that succession in moors 
and in forest burns had been a matter of observation and comment for many 
centuries, the earliest recorded work that approaches investigation in its 
nature was that of King (1685:950) on the bogs and loughs of Ireland. The 
following excerpts indicate the degree to which he understood the nature and 
origin of bogs : 

"Ireland abounds in springs. Grass and weeds grow rapidly at the out- 
burst of these. In winter, these springs swell and loosen all the earth about 
them; the sward, consisting of the roots of grasses, is thus lifted up by the 
water. This sward grows thicker and thicker, till at last it forms a quaking 
bog. ... I am almost (from some observations) tempted to believe that the 
seed of this bog moss, when it falls on dry and parched ground begets the 
heath. . . . It is to be observed that the bottom of bogs is generally a kind 
of white clay or rather sandy marl, and that bogs are generally higher than 
the land about them, and highest in the middle. . . . The true origin of bogs 
is that those hills that have springs and want culture constantly have them : 
wherever they are, there are great springs. 

"I must confess there are quaking bogs caused otherwise. When a stream 
or spring runs through a flat, if the passage be not tended, it fills with weeds 
in summer, trees fall across it and dam it up. Then, in winter, the water 
stagnates farther and farther every year, till the whole flat be covered. Then 
there grows up a coarse kind of grass peculiar to these bogs; this grass grows 
in tufts and their roots consolidate together, and yearly grow higher, in so 
much that I have seen of them to the height of a man. The grass rots in 
winter and falls on the tufts, and the seed with it, which springs up next year, 
and so still makes an addition : Sometimes the tops of flags and grass are 
interwoven on the surface of the water, and this becomes by degrees thicker, 
till it lies like a cover on the water ; then herbs take root in it, and by a plexus 
of the roots it becomes very strong, so as to bear a man. These may be easily 
turned into a meadow, as I have seen several times, merely by clearing a 
trench to let the water run away. Trees are found sound and entire in them, 
and those birch or alder that are very subject to rot. I have seen some of the 



EARLY INVESTIGATIONS 9 

trees half sunk into the hogs and not quite covered. They are generally found 
at the bottom, not only of the wet, but even of the dry red bogs. ' ' 

Degner, 1729. — Degner's dissertation upon peat-bogs, especially those of 
Holland, appears to have been the first comprehensive treatise upon this 
subject, though he cited Schook's "Tractatum de Turns" (1658), and Patin's 
"Traite de Tourbes Combustibles" (1663), as still earlier works. Degner 
combated the assumption that "moss is formed of decayed wood" by the 
following arguments : 

"1. It is contrary to the common opinion of the inhabitants of Holland. 

"2. Trees are not found in every moss. 

"3. Trees are often found buried where no moss is formed. 

"4. Where trees abound are the fewest mosses. They seem rather to 

retard than expedite the formation of mosses. 
"5. Some mosses are found to be 30 feet deep before we reach the wood ; 
it seems incredible that such immense quantities of that matter 
could be formed of wood. 
"6. If forests are converted into moss, the greatest part of Muscovy, 
Tartary and America, and other woody uncultivated regions, 
would, long ere now, have undergone that change, which is not 
the case." 
Degner described the peat-bogs of Holland minutely, and asserted that they 
are often renewed when dug. He stated that the pits and ditches are filled 
with aquatic plants, and that these are converted into peat. He found also 
that when a large pit was dug, and a large sheet of water was left exposed to 
the winds, the growth of aquatic plants was retarded and the renewal of the 
moss checked ; while in small pits aquatics developed rapidly and the renewal 
of the moss was correspondingly rapid. He mentioned as well-known facts 
the filling of a ditch 10 feet wide by 7 feet deep by aquatic plants in 10 to 30 
years to such a degree that men and cattle could safely pass over it, and the 
digging of peat where a navigable lake once existed. 

Buff on, 1742. — Buff on seems to have left the first clear record of the success- 
sion of forest dominants, and of the effect of light and shelter on the process : 

"If one wishes to succeed in producing a forest, it is necessary to imitate 
nature, and to plant shrubs and bushes which can break the force of the wind, 
diminish that of frost, and moderate the inclemency of the seasons. These 
bushes are the shelter which guards the young trees, and protects them 
against heat and cold. An area more or less covered with broom or heath is a 
forest half made; it may be ten years in advance of a prepared area. (234) 

"The best shelter in wet soil is poplar or aspen, and in dry soil Rhus, for 
the growth of oak. One need not fear that the sumac, aspen or poplar can 
injure the oak or birch. After the latter have passed the first few yeai'S in 
the shade and shelter of the others, they quickly stretch up, and suppress 
all the surrounding plants. (237, 238). 

"The oak and beech are the only trees, with the exception of the pine and 
others of less value, that one can sow successfully in wild land." (245) 

Biberg, 1749.— Biberg (1749 :6, 27) described in brief form the origin of a 
meadow from a swamp, and indicated the general stages of succession. Sphag- 
num spread over the swamp until it filled the latter with an extremely porous 
stratum. Scirpus caespitosus then extended its roots into this, and together 
with species of Eriophorum formed elevated peat areas. These furnished a 



10 GENERAL HISTORICAL SUMMARY 

firmer foundation for other invading plants until the whole marsh was con- 
verted into a meadow, especially if the water fashioned for itself a broader 
outlet. He also considered crustose lichens to be the first foundation of vege- 
tation. When the land first emerged from the sea, minute crustose lichens 
began to clothe the most arid rocks. At length they decayed and formed an 
extremely thin layer of earth on which foliose lichens could live. These in 
turn decayed and furnished humus for the growth of mosses, Hypnum, 
Bryum, and Polytrichum, which finally produced a soil on which herbs and 
shrubs could grow. 

Anderson, 1794. — Anderson's views upon the origin and nature of peat- 
bogs may be gained from Bennie (1810:60, 83), who regarded many of them 
as unconfirmed. He considered moss (moor) to be a plant sui generis, which 
continued to increase to an immense magnitude and indefinite age, but that, 
in its progress, it enveloped trees and every other matter that came in its way. 
He reached the conclusion that "nothing can be so absurd, nothing so con- 
tradictory to reason, and every known fact respecting the decomposition of 
vegetables, than the whole of the doctrine that has been implicitly adopted 
respecting the formation of moss, by means of decaying sphagnum or any 
other plant whatever." In support of this, he advanced the arguments 
that : 

' ' 1. All vegetable substances, when dead, decrease in bulk so much that 
they occupy not above one hundredth part of the space they did. 
"2. Moss produces few vegetables; these tend to decay rapidly. 
"3. The vegetable substance which forms moss must therefore have been 

one hundred times more bulky than the moss itself. 
"4. Mosses are found 30, even 40 feet deep. 

"5. The most abundant crop on the most fertile soil will not cover the 
earth, when fresh cut, half an inch deep ; when rotten, it only 
covers the earth one hundredth part of this. 
' ' 6. Therefore, it would require 9,600 years to form a moss 20 feet deep 

on the most fertile soil. 
1 ' 7. Moss produces not one hundredth part of the crop of a fertile soil ; 
therefore, it would require upwards of 900,000 years to product 
20 feet of moss earth on such a soil. ' ' 
De Luc, 1806. — From the various accounts of his investigations furnished 
Rennie by letter, De Luc (Rennie, 1810:137, 128, 116, 30) may well be 
regarded as the keenest and most indefatigable of early students of peat-bogs, 
prior to Steenstrup at least. He was probably the first to make use of the 
term succession, and certainly the first to use it with full recognition of its 
developmental significance. His description of the method by which "lakes 
and pools are converted into meadows and mosses" is so complete and 
detailed that frequent quotation can alone do justice to it: 

"A third kind of peat ground has attracted my attention in the survey I 
took of Brendeburg, Brunswic, and Shleswig: It is connected with lakes. 
The bottom of every dale is a meadow on a subsoil of peat ; this, by gradually 
advancing into, contracts the original extent of the lakes; and, it is well- 
known in that country, that many large lakes have been converted into 
smaller ones, by the peat advancing from the original shores, and many 
places now meadows, and only traversed by a stream, had still a lake in them, 
in the memory of old people. 



EARLY INVESTIGATIONS 11 

"I have said that the peat gradually extends forward in these lakes, con- 
tracting their surface. This is occasioned by the following causes. The 
sandy sediment carried into these lakes by streams gradually raises the 
bottom of them. The consequence of this shallowness is the growth of common 
reeds; these are like the van in the progress; these advance forward as the 
bottom of the lake is raised. No peat appears among the reeds, nor even 
among the small aquatic plants which form a zone behind them. 

"2. Behind the zone of reeds, another rises up. It is distinct from the 
former and it is composed of different aquatic plants, as follows: Scirpus 
maritimus, 8. caespitosws, S. pauciftorus, Equisetum palustre, E. fluviaiile, 
Eriophorum polystachyon, and E. vaginatum; the last of which retains its 
form and appearance longest in the remote peat. 

"3. Behind this zone, the conferva begins to embrace those plants with 
its green clouds ; this forms the bed in which the different species of aquatic 
sphagnum grow ; these thicken the matting, and favor the growth of common 
moss plants, on the compact surface. 

"4. Behind this, another zone appears; it consists of the same kind of 
plants; but these are so interwoven that the surface is more compact and 
bears more weight, though very elastic. On this zone some grasses appear. 

"5. Proceeding backward from this zone, the surface becomes more and 
more compact; many kinds of land plants begin to grow over it, especially 
when that surface, by being raised, is dry in summer. There the Ledum 
palustre, Vaccinium oxycoccon, Comarum palustre, Erica tetralix, and various 
kinds of grasses grow. Thus begins a zone on which cattle may pasture in 
the summer. 

"6. From the beginning of this useful zone, still backward the ground 
becomes more and more solid. This is the last zone that can be distinguished 
by a decided difference in progress. 

"I have said before, that the succession of these different zones, from the 
border of water towards the original border of sand, represents the succession 
of changes that have taken place through time in each of the anterior zones, 
so that, in proportion as the reeds advance, new zones are forming behind the 
advancing reeds, on the same places which they thus abandon. That process 
is more rapid in lakes which are originally shallower, and slower in deeper 
lakes. It seems even to be stopped in some parts, where the reeds, which can 
not advance beyond a certain depth, approach the brow of a great declivity 
under water ; there the progress, if continued, is not perceptible : But in lakes 
originally not very deep, and in which the sandy sediments are advancing all 
around, the reeds, forming a ring, gradually contracting its circumference, 
meet in the center ; and at last these reeds themselves vanish, so that instead 
of a lake, a meadow occupies its surface. In some of these meadows, attempts 
have been made to keep up a piece of water, but the attempt is vain, excepting 
at a great expense : for luxuriant aquatic plants soon occupy that space, and 
the peat, advancing rapidly, restores the meadow. ' ' 

De Luc also noted the significance of wet and dry periods in the develop- 
ment of the bog : 

1 ' The surface of these pits is covered with all kinds of ligneous and aquatic 
plants that delight in such a soil; these alternately overtop each other; the 
ligneous plants make the greatest progress in a dry summer, so that the 
surface seems to be entirely covered with them. The reverse is the case in a 
rainy summer. The aquatic plants overtop the ligneous and choke them 
insomuch that the whole surface seems to be entirely covered with a matting 
of aquatics which, by decaying, form a soil for the ensuing season. If it 



12 GENERAL HISTORICAL SUMMARY 

continues rainy for a succession of years, these aquatic plants continue to 
prevail till a dry season comes. This is so certain, that in the succession of 
beds, or strata of the moss, these different species of plants are distinguish- 
able. These strata are either composed of the roots and fibres of ligneous 
plants, or of the remains of aquatic ; so that upon examining some of the cuts 
of the deepest canals, one saw distinctly the produce of the several years, and 
could even distinguish the different produce of a wet and dry season, from 
the residuum each had left." 

Rennie, 1810. — Rennie, in his "Essays on the Natural History and Origin 
of Peat Moss," gave the first comprehensive and detailed account of peat- 
bogs. His book is an almost inexhaustible mine of opinions and observations 
from the widest range of sources. It must be read in detail by everyone who 
wishes to be familiar with the beginnings in this most important part of the 
field of succession. The titles of the nine essays are as follows : 

I. Of Ligneous Plants. 
II. Of Aquatic Plants. 

III. On the Changes and Combinations by which Vegetable Matter is converted 

into Moss. 

IV. On the Simple and Compound Substances that may be Expected and are 

EeaJly Found in Peat Moss. 
V. On the Alliance Between Peat, Surtur-brandt, Coal, and Jet. 
VI. On the Alliance between Peat and Other Bituminous Substances. 
VII. On the Distinguishing Qualities of Peat Moss. 
VIII. On the Sterility of Moss in its Natural State, and Causes of it. 
IX. On the Different Kinds and Classification of Peat Moss. 

Rennie discussed at length the relation of forest to peat-bogs, and stated 
that in many bogs one tier of roots appears perpendicularly above another, 
while in some even three tiers appear in succession. Trees are also found 
growing upon the ruins of others after they have been converted into moss. 
He cited the observations of the Earl of Cromarty with reference to the 
replacement of forest by bog: 

"That, in the year 1651, when he was yet young, he visited the parish of 
Lochbroom in West Ross ; that he there saw a small plain covered with a 
standing wood of fir trees, which were then so old that they had dropped 
both leaves and bark. On a visit to this forest 15 years afterwards, not a 
tree was to be seen, and the whole plain was covered with green moss. By 
the year 1699, the whole had been converted into a peat moss from which 
the inhabitants dug peat. ' ' 

The author quoted many opinions upon the secondary development of peat 
when the original deposit had been dug, and concluded that the conditions 
requisite for regeneration were that the pits be full of water, and that the 
water be stagnant. The process went on most rapidly in small pits with 
shallow water. A resume of opinions upon the rate of peat formation was also 
given, and extensive extracts from De Luc, Poiret, Degner, Anderson, "Walker, 
and others were commented upon. As to the vegetation of mosses, he con- 
cluded that many peat -bogs, when dug, are renovated by aquatic plants ; that 
the same species of plants have contributed and still contribute to the original 
formation of many mosses ; that many lakes in the north of Europe have been 
converted into moss and then into meadows by the growth of these or similar 
aquatic plants ; that aquatic plants may be traced in most, if not all, moss ; 
and that many fertile plains, in the course of ages, have undergone changes 



EARLY INVESTIGATIONS 13 

from arable lands to forests, from forests to lakes, from lakes to mosses, from 
mosses to meadows, and from meadows to their original state of arable land. 
He likewise supposed that many low levels, covered with wood, had been con- 
verted into morasses. In citing examples of such changes, he also made use 
of the term "succession," in the following sentence: (227) 

"The first is Low Modena, which seems to have undergone all these changes ; 
the second is the bog of Monela in Ireland, which seems to have been sub- 
jected to a similar succession. Carr, in his 'Stranger in Ireland' (1806:190) 
says: 'Stumps of trees are still visible on the surface of the bog of Monela; 
under these lies a stratum of turf 10 or 15 feet deep ; under this a tier of 
prostrate trees is discovered ; beneath these another stratum of earth is found 
of considerable depth ; and below this a great number of stumps of trees are 
found, standing erect as they grew. Thus, there is a succession of three dis- 
tinct forests lying in ruins, one above the other.' (229) There are other cir- 
cumstances which render it equally probable that one generation has risen 
upon the ruins of another. In many mosses one tier of roots appears perpen- 
dicularly above another ; yet both are fixed in the subsoil. In some even three 
tiers appear, in succession, the one above another." (27) 

Dureau, 1825. — Dureau de la Malle (1825:353), attracted by the work of 
Young on the effect of rotation upon crops, endeavored to trace the same 
principle in woodland and meadow. As a landed proprietor in Perche, he 
possessed unusual advantages for this purpose, both in the utilization of the 
forests and in experiments designed to prove that the alternative succession 
of plants is due to the long retention by seeds of the power of germination. 
In cutting the woods of Perche, composed of Quercus, Fagus, Castanea, 
Vlmus, and Fraxinus, only oaks and beeches were left as seed trees. The cut- 
over areas came to be occupied by Genista, Digitalis, Senecio, Vaccinium, 
and Erica, and finally by Betula and Populus tremula. At the end of 30 
years, the birch and aspen were cut, and quickly succeeded themselves. The 
oak and beech returned only after the third cut, 60 years later, and became 
masters of the area. Since there were no adjacent aspens and birches, the 
author believed their seeds could not have been brought by the wind, and he 
concluded that the seeds remained dormant in the soil for at least a century. 
He noted also the reappearance of rushes, sedges, and grasses in clear areas 
in the heath, and stated that he had observed the grasses and legumes of a 
natural meadow successively lose and gain the preeminence for five or six 
times in 30 years. The results of his observations and experiments are 
summed up as follows: 

"The germinative faculty of the seeds of many species in a large number of 
families can be retained for 20 years under water, or for at least a hundred 
years in the soil, provided they are not subject to the action of atmospheric 
factors. 

"The alternance or alternative succession in the reproduction of plants, 
especially when one forces them to live in societies, is a general law of nature, 
a condition essential to their conservation and development. This law applies 
equally to trees, shrubs, and undershrubs, controls the vegetation of social 
plants, of artificial and natural prairies, of annual, biennial, or perennial 
species living socially or even isolated. This theory, the basis of all good 
agriculture, and reduced to a fact by the proved success of the rotation of 
crops, is a fundamental law imposed upon vegetation." 



14 GENERAL HISTORICAL SUMMARY 

Steenstrup, 1842. — Steenstrup (1842:19) was the first student of peat-bogs 
to turn his attention to the succession of fossil horizons preserved in the peat. 
His pioneer work is the classic in this much-cultivated field, and since it is 
practically inaccessible, a fairly full abstract of it is given here. The memoir 
consists of five parts, viz., (1) Introduction; (2) Description of Vidnesdam 
Moor; (3) Description of Lillemose Moor; (4) Comparative development of 
Vidnesdam and Lillemose Moors; (5) General observations upon the Tree-, 
Scrub-, and Heath-moors of Denmark. It is chiefly the detailed descriptions 
and comparison of the moors which are summarized in the following pages : 

The bottom of Vidnesdam consists of a layer of bluish clay, containing 
leaves of a grass and of Myriophyllum and fruits of Cham. Above this lies 
a layer of fresh-water lime, inclosing a very large number of leaves of 
Potamogeton ootusifolius zosterifolius, and perhaps of Sparganium natans. 






Fiq. 1. — Section of Vidnesdam moor, showing various layers of 
the cosere. After Steenstrup. 

The leaves and stems are incrusted with lime, and are stratified in this layer, 
in which Chara and Myriophyllum also occur. An interruption in the forma- 
tion of the lime layer is indicated by a lamina of Hypnum fluitans and 
Myriophyllum verticillatum. In the cross-section of the bottom of the moor 
(fig. 1), these three layers are designated by m, n, and o respectively. The 
best series of layers, however, is the marginal one, which follows the slopes 
all around the moor. The drift c is covered by a layer of cones, needles, and 
branches of conifers, 1 to 1.5 feet in depth. In this are embedded large 
coniferous roots, the trunks of which lie in the spongy peat layers toward the 
center. The large number of trunks found upon a small area leads to the 
conclusion that the pine (Pinus silvestris) grew in a dense, pure stand. The 
pine trunks found in this layer r extend into a layer of peat which lies 
directly above the lime layer n. The lower part of the peat layer is filled with 
grass-like leaves, but the upper part consists wholly of Sphagnum. Above, 
the latter is mixed with Hypnum cordifolium, which finally becomes predomi- 
nant and forms the layer q. The position of the Sphagnum below and about 
the pine trunks indicates that this layer must have been forming before as 
well as at the time of burial of the trees, while the Hypnum layer must have 
developed subsequently. Pine roots also occur in this layer, but the pines to 
which these stumps belonged must have grown at a later period and under 
much less favorable conditions than those of the forests preserved in layer r. 
An oak period must have followed that in which these stunted pines grew, 
as oak trunks occur directly above layer q. Oak leaves and fruits were rare 
about the trunks, but on the marginal slopes remains of the oak (Quercus 
sessiliflora) dominate the layer s. They become recognizable only with diffi- 
culty in the upper part of the layer, which then passes gradually into an alder 
layer t. The latter is the top layer of the moor, covering the oak one to a 



EARLY INVESTIGATIONS 



15 



depth of 3 to 4 feet, both at the margin and in the center. Oaks occur occa- 
sionally in this layer, though the alders are wholly predominant, their 
branches, leaves, and catkins sometimes forming the peat alone. The large 
number of nuts indicates that hazel (Corylus avellana) probably formed a 
considerable portion of this layer, especially near the margins. In the north- 
ern portion of the moor the Hypnum layer contained leaves of Eriophorum 
angusti folium, and scattered trunks of Betula. 







Fig. 2. — Section of Lillemose moor, showing central and marginal layers of the 
cosere. After Steenstrup. 



In the Lillemose Moor, the structure is indicated by the cross-section shown 
in figure 2, in which the following layers are seen, from below upward : 

Central Area. Margins. 

V. Sphagnum with alder t. Alder. 

u. Hypnum proliferum with remains of birch and oak. 

q. Sphagnum with oak, above with Oxycoccus, Eriophorum, etc s. Oak. 

p. Hypnum cordifolium with pine and some aspen r. Pine. 

n. Silica layer with Potamogeton and aspen c. Drift. 

d. Sandy clay, the substratum d. Sandy clay. 

The lower part of layer n seems almost a continuation of d, but the upper 
portion clearly shows the remains of Hypnum cordifolium, Potamogeton, 
Equisetum, Myriophyllum, Alisma, and especially leaves and twigs of Populus 
tremula everywhere in the layer, showing that the latter grew upon the mar- 
ginal slopes. This foliated silica layer is covered by a peat layer of Hypnum, 
p, which is also in direct contact with the substratum over some parts of the 
banks. Pine needles and cones occur with the Hypnum, and on the margin 
become so abundant as to form a layer r, which consists almost wholly of pine 
cones, needles, and bark, mixed with some Hypnum fluitans. The pine layer 
also contains remains of Betula, Salix, and Menyanthes. Above the pine 
stratum lies the oak layer, containing twigs, leaves, acorns, cups, and an occa- 
sional trunk of Quercus sessiliflora. The next layer is that of alder peat, com- 
posed almost wholly of Alnus glutinosa, but with an occasional Betula or Salix. 
This uppermost layer covers the entire surface of the moor as well as the 
margins. In the center of the moor, the layer n is covered by Hypnum peat, p, 
which is pure below, except for roots of Nymphaea, leaves of Populus and 
Salix, and fruits of Betula. In the upper part occur pine leaves and trunks. 
The next layer, q, is composed of Sphagnum, with Oxycoccus vulgaris, Andro- 
meda polifolia, Scirpus caespitosus, and Eriophorum angustifolium in the 
upper portion, together with some oak and birch. This is followed by a layer u 
of Hypnum proliferum, with remains of both oak and birch, and this is in turn 
covered by an extensive layer of alder peat f. 



16 GENERAL HISTORICAL SUMMARY 

The peat layers of the two moors not only afford a record of the successive 
populations which occupied the basin, but also of the different forests which 
clothed its margins. In the basin proper, Vidnesdam shows but two strata of 
moss peat, namely, the Sphagnum and the Hypnum cordifolium layers, while 
in Lillemose there are three strata and in the reverse order, viz., Hypnum 
cordifolium, Sphagnum, and Hypnum proliferum. As to the margins, it is 
assumed that the banks were without vegetation during the period in which 
no plants had appeared in the water of the basin. With the early stages of 
water vegetation, forest seems to have appeared on the banks, for the quantity 
of aspen leaves found in layer n shows that this tree must have been domi- 
nant. These give way about the middle of layer p to abundant pine needles, 
indicating that the aspens had been replaced by pine, as would be expected 
in the normal succession. The marginal layer r testifies to the length of the 
period for which the pine dominated the margin, but it finally yielded to the 
oak, as is shown by the plant remains in layer q. The marginal layer 5 is 
perhaps due wholly to the oak forest, but this seems to have been destroyed 
by increasing moisture, resulting in a Hypnum layer, which was finally suc- 
ceeded by Sphagnum and alders. 

There is thus no doubt that these two moors have developed during a period 
in which several forest vegetations have arisen and disappeared. The aspen 
forests may be regarded as preparatory to the pine and oak forests, which 
probably dominated the region for thousands of years, but have practically 
disappeared from the country to-day. While these forests, as well as the moor 
vegetations, belong in a definite time sequence, it is practically impossible 
to assign any absolute time for any or all of the layers. 

The four forest vegetations, viz., aspen, pine, oak, and alder, found above 
each other in Vidnesdam and Lillemose, occur in all the forest moors of north 
Sjelland, and other evidence points to their former occurrence throughout 
Denmark. These four forests not only succeeded each other in the moors, 
but everything points to a synchronous succession on the uplands, so that one 
may speak of a pine period, for example, for the whole country. The final 
alder forest of the moor was succeeded by the beech forest which is now the 
dominant one. However, no trace of the beech has been found in the moors. 
Thus there seems no doubt that one vegetation succeeeded another in such a 
way that the later grew on the ruins of the former, and that the replacement 
of one by the other was the result of a slow natural cycle. In this cycle one 
organization develops and then gradually calls forth conditions which cause 
its disappearance and hasten the development of a new organization. 

Eeissek 1856. — Reissek (1856:622) studied in detail the formation and 
succession of islands in the Danube. These arose through separation from 
the mainland, or through the deposition of gravel and sand. It was thought 
that islands due to deposition were laid down irregularly and without sorting, 
and that their formation bore no direct relation to the development of vege- 
tation. The author found the process of formation both definite and regular, 
and the influence of the vegetation fundamental. Each island was at first a 
sand-bar due to high water or ice action. The first vegetation consisted of 
scattered willows, most frequently Salix purpurea. The willows became 
bushy and caught the water-borne sand, building hummocks which gradually 
united to form a sandy level 6 to 8 feet above the gravel. The willows them- 
selves came to be half-buried in the sand. All other invaders established 
themselves in the sand among the willow crowns. They entered in a definite 
succession, so that a sequence of stages results, each with its characteristic 



EARLY INVESTIGATIONS 17 

woody plants. Salix purpurea, S. riparia, and Myrica germarvica belonged 
solely to the first stage. The second stage consisted of Alnus incana, Popuhis 
alba, and Cornus sanguinea, and the last stage of Fraxinus excelsior, JJlmus 
campestris, Acer campestre, Quercus pedunculata, Pirus malus, P. communis, 
etc. High water and drift-ice often destroyed young islands entirely or 
partly, exposing the gravel-bank on which the sere might be repeated. Par- 
tial destruction of the sandy plain permitted the development to begin again 
in new areas alongside of those in later stages. The pioneer willows died off 
as soon as the trees of the second stage developed much shade, a fate which 
also overtook the groups of Phragmites which occurred among the willows. 

Vaupell, 1857. — In discussing the invasion of the beech into Denmark, 
Vaupell (1857:55) reviewed the evidence obtained from submerged forests, 
deposits of calcareous tufa, and peat-bogs. The ancient forests of Denmark, 
and especially of Jutland, were a mixture of coniferous and deciduous species. 
Betula was the most common, with Quercus and Pinus silvestris next in im- 
portance; the aspen, willow, hazel, elm, and maple played but a secondary 
part. In existing forests Fagus is the universal dominant. Since remains of 
the beech are lacking in peat, tufa, and in the submerged forests, Vaupell 
concluded that it had entered Denmark at a subsequent time. In seeking an 
explanation of the change of dominance, he cited the opinions of Dureau de la 
Malle, Laurent, and Cotta in favor of the natural "alternation of essences," 
but reached the conclusion that it must be produced by other causes than the 
exhaustion of the soil. Where the beech invades forests of birch, it gains the 
upper hand by overshadowing the birch trees, suppressing them and causing 
their death. The birch fails also to reproduce because its seedlings do not 
thrive in the dense shade of the beech. The plantations of pine are likewise 
invaded by the beech with similar results, unless protected by man. In the 
cases where beech has yielded to pine, the explanation is always to be found 
in intervention by man. The author concluded that the beech had migrated 
from its center in France and Germany during the present geological period, 
establishing itself wherever the soil became drier or richer, and dispossessing 
the birches and pines everywhere but in marshy or sterile soil. 

von Post, 1861. — von Post (1861) appears to have been the first to give a 
complete and detailed account of the reactions by which plants and animals 
produce soils. Ramann (1888) has summarized his work upon the copro- 
genous formation of the various biogenous soils. Muck (Schlamm, gyttja) 
consists of plant fragments, including diatom shells. It forms very elastic 
masses which are deposited on the bottom in waters, springs, brooks, lakes, 
etc. Muck is formed by the deposition of insect excreta, together with the 
remains of dead infusoria, Crustacea, and insects, diatom shells, and alga?. 
Such muck deposits are often found beneath peat moors; "Lebertorf" is a 
kind of fossil muck. Moor soil is deposited more rapidly than muck in waters 
colored brown by humus material. Moor soil consists of a dark-brown, soft 
mass which dries into a hard mass with extreme shrinkage, which is then no 
longer plastic in water. It consists of finely divided plant remains arising 
from the excrement of water animals, particles of humus material, and, for 
the remainder, of the same materials as muck. The animal excrement, how- 
ever, is more abundant, the diatoms less. Moor soil is formed chiefly in lakes 
and ponds in forests, when they contain much humus material in solution 



18 GENERAL HISTORICAL SUMMARY 

which is precipitated by lime salts. Peat consists of brown organic masses of 
plant remains which have not been eaten. It is deposited in a mass consisting 
predominantly of animal excrement, and contains diatoms and animal remains 
in small degree. Peat arises in waters which are more or less clothed with 
aquatic plants. Besides the common grass-peat, the moss-peat of the conif- 
erous forests is characteristic for Sweden. In ponds containing Calla and 
Menyanthes there develops a vegetation of Sphagnum, upon which later 
Calluna and Ledum, as well as spruces and pines, establish themselves. In 
more northern regions, lichens overgrow the moss-peat, especially Cladonia 
rangiferina and Biatora icmadophila. A peculiar kind of peat is carr-peat, 
which consists of the roots of sedges, Calamagrostis arctica, Deschampsia flex- 
uosa, etc. Mull or humus consists of digested plant-parts and animal remains, 
together with brown granular amorphous particles, which are to be regarded 
as precipitates of humus salts. These are insoluble in water, acid, and alkali. 
Between these constituents occurs an equal amount of animal excrement. 
The various kinds of humus are moss and lichen humus which consist pre- 
dominantly of animal remains, coniferous forest humus consisting of decom- 
posed wood, mycelia, etc. Deciduous forest humus, darker than the fore- 
going, is rich in excrement and animal remains, and contains much humic acid. 
Grass-humus consists chiefly of animal excrement mixed with sand and clay. 

Gremblich, 1876.— Gremblich (1876, 1878:1014) called attention to the 
succession in a particular area of different formations, each of which pre- 
pared the way for the following one: 

"We see certain formations invade an area, dominate it for a while, and 
then disappear, until finally the rotation of formations falls into inactivity, 
in order perhaps to begin a new cycle which takes the same course. If one 
follows the course of succession in a moor, he will notice that in general three 
clearly marked phases may be distinguished. The first phase has for its task 
the preparation of the bare ground for vegetation. The second is marked by 
a cover which shows great luxuriance, both of species and individuals. In 
the last phase appears a plant covering which closes the rotation of organic 
life, and marks the death of the succession. The last two stages as a rule 
store up carbon dioxid in some form, e. g., wood, peat or humus. Each suc- 
cession in a particular area shows close relationship with that of the moor, 
and the development of the latter may be taken as the type for all successions. 
We venture to say that moor succession or some parallel development takes 
place wherever man leaves nature to her own course. ' ' 

Gremblich also described the invasion of talus in the Dolomites of the 
Tyrol, and pointed out the three successive phases of development. The first 
phase was marked by lichens and low herbs, Thlaspi, Galium, Saxifraga, 
etc. The humus thus produced was invaded by Adenostyles, Ranunculus, Saxi- 
fraga, Rhododendron, Rosa, Rhamnus, Crataegus, Alnus, and Pinus, as the 
most important species of the second phase. The last phase was marked by 
the entrance of Sphagnum, or of Nardus, Scirpus caespitosus, Azalea procum- 
lens, Empetrum nigrum, etc., which form peat, often a meter deep. The last 
plants, Azalea and Empetrum, finally disappear and the naked peat alone 
remains, to be again colonized when soil is drifted upon it by the wind. 

Miiller, 1878-1887.— Midler (1878, 1884, 1887) made a critical investiga- 
tion of the humus soils of beech and oak woods and of heath, which is of the 
first importance for the study of the factors which affect invasion and replace- 



EARLY INVESTIGATIONS 19 

ment in forests. The soil of the beech forest is distinguished as of two types. 
In the first, the surface is covered with a layer of leaves and twigs which build 
an incoherent mass. This covers the upper soil, which consists of loose earth, 
and is 3 to 5 feet or more deep. Sometimes the entire upper soil is dark gray- 
brown, but frequently only the uppermost layer has this color. The latter 
is then called mull. It has a characteristic ground-cover of Asperula, Mercuri- 
alis, Milium, Melica, Stellaria, Anemone, etc. It is defined by Miiller as 
follows: "Beech mull is a loose incoherent layer of converted leaves, twigs, 
etc., of the beech forest, rich in animal life and with the organic material 
intimately mixed with the mineral earth. ' ' In the second type, the leaf litter 
is lacking. The soil is firm, filmy, and absorbs rain like a sponge. The upper 
part is composed of a tenacious brown-black layer of humus. The vegetation 
consists characteristically of Air a, Trientalis, Maianthemum, Potentilla, etc., 
and many mosses. The beech thrives poorly in contrast with its growth in 
beech mull, and the old trees are mostly in a pathological condition. Beech 
turf is regarded as consisting of a leaf-mold of the beech woods which is poor 
in animal life ; it is united into a firm peat by roots and by a very persistent 
mycelium. It is significant that the reproduction of beech upon mull is easy, 
while upon peat young trees can not come to maturity. This indicates that 
the peat was formerly clothed with mull. If a beech forest upon mull is com- 
pletely cut off so that no beech peat is naturally formed, there appears 
another vegetation which in its turn prepares the soil and opens the way for 
other forms. The mull may retain its essential character or may be converted 
into heath peat. After the destruction of beech forest upon beech peat, no 
new forest can appear, as a rule, but the soil is densely clothed with Aira 
flexuosa, and the peat layer is finally destroyed by the grass. In similar 
thorough fashion, the author considered the soil of oak woods and of heath in 
reference to the changes in them which affect the succession. 

Other investigations. — From 1802 to 1885, when Hult's classic work upon 
the developmental study of vegetation was published, there appeared a large 
number of works in which succession was treated more or less incidentally. 
These dealt mostly with peat-bogs, or with succession after fire or disturbance 
by man. Among the former were the important monographs or handbooks of 
Eiselen (1802), Dau (1823), Wiegmann (1837), Lesquereux (1844), Grisebach 
(1845), VaupeU (1851), Lorenz (1853, 1858), Pokorny (1858, 1860), and 
Senft (1861, 1862). The others may be mentioned briefly here. De Candolle 
(1820:27) mentioned the cultures on the dunes of the "Landes," in which 
the rapidly growing Genista, after having served as cover for seedlings of 
Pinus, was finally driven out by the latter. Lund (1835) and Reinhardt 
(1856) studied the origin of the Brazilian campos or savannahs, the former 
concluding that they had been derived from forest as a consequence of fire, 
while the latter regarded the effect of fire as secondary. Berg (1844) studied 
the successive modifications of the deciduous forests of the Harz in connection 
with their disappearance before the conifers. He showed that the forests 
remained unchanged just as long as they were undisturbed by man, and that, 
while trees with winged migrules readily invaded wind-throw areas, they 
were gradually replaced by the species of the surrounding forest, Humboldt 
(1850:10) dealt with succession only incidentally, though lie clearly recog- 
nized it as universal: 



20 GENERAL HISTORICAL SUMMARY 

"In northern regions, the absence of plants is compensated for by the 
covering of Bceomyces roseus, Cenomyce rangiferinus, Lecidea muscorum, 
L. icmadophila and other cryptogamia, which are spread over the earth and 
may be said to prepare the way for the growth of grasses and other herba- 
ceous plants. In the tropical world, some few oily plants supply the place of 
the lowly lichen." (125) "Thus one organic tissue rises, like strata, over the 
other, and as the human race in its development must pass through definite 
stages of civilization, so also is the gradual distribution of plants dependent 
upon definite physical laws. In spots where lofty forest trees now rear their 
towering summits, the sole covering of the barren rock was once the tender 
lichen; the long and immeasurable interval was filled up by the growth of 
grasses, herbaceous plants, and shrubs. ' ' 

Henfrey (1852:56) considered briefly the changes in vegetation due to man : 

"It is certain that the appropriate stations of many plants would be 
destroyed with the removal of forests, and new conditions of soil created for 
the habitation of immigrants from other regions. But the modification of the 
surface so as to alter the physical condition of the soil is by far the most 
important change brought about in reclaiming land for cultivation. The 
banking out of the sea changes by degrees the vegetation of its shores; bare 
sand-dunes, where scarcely a plant could maintain a precarious footing, are 
by degrees covered with vegetation; sandy inland wastes are rescued from 
the heath and furze, and made to contribute at first by coniferous woods, 
such as the larch, and when the soil has become by degrees enriched, by the 
plants requiring a better nourishment, to the general stock of wealth ; and in 
these changes many species are destroyed, while others naturally making 
their way into a fitting station, or brought designedly by the hand of man, 
grow up and displace the original inhabitants." 

De Can doll e (1855:472) cited the conclusions of Dureau de la Malle 
(1825), Laurent (1849), and Meugy (1850) as to the "alternation of forest 
essences, ' ' a subject much discussed in the works on forestry of this time. He 
failed, however, to recognize the fundamental nature of succession, for he 
regarded the alternation (succession) of forest dominants as a process dis- 
tinct from that which occurs when a forest is burned or cut. It seems 
probable that the difference he had in mind is that which distinguishes 
primary from secondary succession. Hoffmann (1856:189) found Bubus to 
be the first invader in forest burns in the Ural Mountains; this was followed 
successively by Amelanchier, Alnus, Betula, and other deciduous trees, and 
these were finally replaced by pines and other conifers. Hill (1858) first 
pointed out that the second growth in forest burns or cuttings is normally 
composed of genera different from those found in the original vegetation. 
Stossner (1859) described in detail the conversion of a fallow field covered 
with Viola into a mountain meadow. 

Middendorff (1864:641) considered the succession of dominants to be the 
exception rather than the rule in the case of burn forests in Siberia, and 
explained the cases in which other species replaced the original forest domi- 
nants as due to the influence of man. Kabsch (1865 :75) pointed out the pri- 
mary role of lichens in succession on rock surfaces: 

"Lichens are the real pioneers in vegetation; they corrode the hardest 
basalt as they do the softest limestone, decompose the rock, and mix its 
particles with their own remains, in such a way as to give opportunity for the 
growth of a higher vegetation." 



RECENT INVESTIGATIONS 21 

Engler's pioneer work (1879) upon the developmental history of vegetation 
deals primarily with the geological development and the relationship of floras, 
but has little bearing upon succession. Nathorst (1870, 1873) was the first 
to demonstrate the presence of arctic plants, Salix herbacea, S. polaris, S. 
reticulata, and Dryas octopetala, in beds of postglacial clay in southern 
Sweden. These and other arctic species were also found at the bottom of 
moors in Seeland. Nathorst discovered Betula nana, Salix retusa, S. reticulata, 
Polygonum viviparum, and Loiseleuria procumbens in layers resting directly 
upon glacial deposits in Switzerland. Salix polaris was also found under the 
glacial boulder clay at Cromer in England, and various other willows between 
the clay and the ' ' forest beds. ' ' 

RECENT INVESTIGATIONS. 

Blytt, 1876.— Blytt (1876, 1881) advanced the theory that since the glacial 
period the climate of Norway has undergone secular changes in such fashion 
that dry periods of continental climate have alternated with moist periods 
of insular or oceanic climate, and that this has happened not once but repeat- 
edly. As long as land connections permitted a mass invasion, continental 
species entered during one period and insular species during the other. This 
theory is supported by investigations of the peat-beds of Norway, the oldest 
of which have an average depth of 16 feet. They consist of four layers of 
peat with three intervening layers of remains of rootstocks and forests. The 
surface of the drier moors is more or less completely covered with heather, 
lichens, and forest. "With increasing moisture, forest and heath disappeared, 
and were replaced by moor, while at the same time species of Sphagnum domi- 
nated the wetter places almost wholly. The root layers, on the other hand, 
represent periods when the moor was drier than formerly, and during which 
peat formation probably ceased for thousands of years, to begin again later. 
In the oldest moors there are traces of three such dry periods, and such moors 
are often covered to-day with forests for the fourth time. 

The explanation of such changes has been sought in local causes, but Blytt 
is convinced that it lies in the alternation of dry and wet periods. When the 
rainfall and humidity changed, the surface of the moor must have become 
drier or wetter in consequence, and have produced the vegetation found in the 
alternating layers of peat and forest remains. The absence of forest beds in 
the wet moors, and their presence only in the dry ones, seem to indicate that 
this has not been produced by local causes. The moors of Norway are at 
present drier than formerly, and are mostly covered with forest or heath, while 
the Sphagnum layer just below the surface indicates that the period just pre- 
ceding was a wetter one. In the second place, Norway has been elevated 
since the glacial period, and the greater depth of peat-beds at high altitudes 
is taken as an indication that the formation of peat began long before the land 
reached its present level. 

The four layers of peat investigated by Steenstrup in Denmark are sepa- 
rated by forest layers which agree with those of Norway. The profile for the 
two countries is as follows: 

1. The present. The moors are mostly dry and contain a new root layer 
ready to be buried under peat deposits as soon as the new moist period begins. 



22 GENERAL HISTORICAL SUMMARY 

2. Peat. Probable period of the invasion of sub-Atlantic flora, apparently 
prehistoric, because stone implements are found in the young layers. 

3. Stumps with forest remains. 

4. Peat with trunks and leaves of Quercus sessiliflora. 

5. Stumps with forest remains, hazel, oak, etc. 

6. Peat with pine trunks. 

7. Stumps and forest remains. 

8. Peat with leaves of Populus tremula and Betula odorata. 

9. Clay with arctic plants, Dryas octopetala, Salix reticulata, Betula 
nana, etc. 

10. Closing stages of the glacial period ; moist climate. 

Blytt's theory has been the storm center of the study of Scandinavian and 
Danish moors. It has been accepted and modified by Sernander (1891, 1894, 
1895, 1899, etc.), and vigorously combated by Andersson (1893, 1896, 1898, 
1903, etc.). Blytt (1892) found further support for his view in an investiga- 
tion of the calcareous tufas of Norway. Johanson (1888), Hulth (1899), 
Holmboe (1904), Lewis (1905-1911), Haglund (1909), Samuelsson (1911), 
and others have studied boreal moors with especial reference to the theory of 
alternating wet and dry periods. 

Hult, 1885-1887.— To Hult belongs the great credit of being the first to 
fully recognize the fundamental importance of development in vegetation, 
and to make a systematic study of a region upon this basis. He maintained 
that the distribution of plant communities could be understood only by trac- 
ing the development from the first sparse colonies upon bare soil or in water to 
the now dominant formations. He also laid down some of the general prin- 
ciples upon which the developmental study of vegetation must be based, and 
was the first to grasp the significance of the climax. In his classic investiga- 
tion of the vegetation of Blekinge in Finland (1885:161), Hult traced the 
succession of each intermediate formation through its various stages to the 
supposed climax. He found that grassland on poor soil became heath; on 
rich soil, oak wood. The heath developed into forest, dominated by Betula 
alone, or mixed with Picea, Pinus, or Quercus. Betula is displaced upon dry 
sandy soil by Pinus, upon moist soil by Picea. The spruce forest reacts upon 
the soil in such a way as to favor the invasion of Fagus, which eventually 
replaces the spruce. The birch forest can also be replaced by oak forest, which 
gradually develops into beechwood. "Where the oak becomes dominant in 
grassland or heath, it develops into a scrub, which appears to yield finally 
to beech scrub. On dry banks, the scrub is replaced by birch, this by spruce, 
and the latter finally gives way before the beech. The Menyanthes community 
of wet banks is followed by Carex, and this by meadow moor, which yields to 
birch forest. The latter in turn is replaced by spruce forest, which seems to 
persist as the climax. The sequence of development in the moor is (1) aquatic 
formation, (2) Carex moor, (3) hummock moor, (4) peat moor, (5) pine moor, 
(6) birch forest, (7) spruce forest. In the swamps, the succession is as follows: 
(1) Potamogeton, (2) Sphagnum-Amblystegium, (3) Menyanthes-Eriopkorum, 
(4) Car ex-Sphagnum, (5) peat moor, (6) birch forest, (7) spruce forest. 

The following were regarded as climax communities, but it seems obvious 
that the beech forest is the only real climatic climax: (1) rock heath, (2) pine 
forest on dry sand or on peat moor, (3) spruce forest on shallow shore moors, 



RECENT INVESTIGATIONS 



23 



(4) birch forest on deep moors, (5) woodland along streams, (6) thorn scrub 
in warm, dry places, (7) beech forest in all other places. The behavior of the 
beech as the climax dominant is the same in Finland that Steenstrup and 
Vaupell have shown for Denmark and Fries for Sweden. Ilult thought that 
this does not indicate a change of climate, but merely the return of the beech 
into areas from which it was largely removed by lumbering. 

Hult (1887:153) also traced the development of the alpine vegetation of 
northernmost Finland. He found that in the drier places Cladineta and Alec- 
torieta finally replaced all other communities, while in moist areas grass and 
herb consocies passed into communities of dwarf shrubs, or even into a lichen 
climax. The development everywhere was marked by a transition from more 
hygrophilous to more xerophilous conditions. The initial stage of succession 
was determined by the local conditions of colonization. The sequence itself 
was regarded as everywhere constant; in no place did a backward develop- 
ment take place. 

Warming, 1891.— Warming (1891, 1-895, 1907) was the first to give a con- 
sistent account of succession on sand-dunes, and his pioneer studies in this 
field have served as a model for the investigation of dune seres in all parts of 
the world. He found that the shifting or white dunes began as heaps of sand 
formed by tides, waves, and wind; the particles as a rule are less than one- 
third of a millimeter in diameter. The further growth of such dunes is made 
possible by sand binders, such as Psamma arenaria, Elymus arenarius, Carex 
arenaria, Agropyrum junceum, Lathyrus maritimus, Alsine peploides, etc. The 
last two are found only on the lower dunes, and are sooner or later driven out 
by Psamma and Elymus, which are especially adapted to the building of high 
dunes, because of their ability to push up through a cover of sand. Psamma, 
however, is the most important pioneer, and excels all others in its ability to 
collect sand among its tufted leaves, and to grow up through it. Other plants 
find their way in among the shoots of Psamma and Elymus, and, as the sand 
becomes more and more fixed, conquer the intervening spaces. The more 
effectively these two grasses fix the soil, the more they prepare it for other 
species, which ultimately replace them. Lichens, mosses, and perennials 
which form tufts or mats, or possess a multicipital primary root, establish 
themselves at this stage, and the dune passes into a stable or gray dune. 

"Warming recognized two principal associations (consocies) among those of 
the shifting dune, viz, Psammetum and Elymetum. Woody species such as 
Hippophae rhamnoides, Salix repens, and Empetrum nigrum appear here and 
there, and give rise to scrub. The gray dune may pass into dune-heath or 
dune-scrub, and then into dune-forest. In the north of Europe may be 
encountered the following formations, which show a zonal succession to some 
extent. It is obvious that the zonal order is essentially that of the develop- 
mental sequence. 

5. Stationary or gray dunes. 

6. Dune-heath and dry sand field. 

7. Dune-scrub. 

8. Dune-forest. 



1. Sand 

2. Iron-sulphur bacteria. 

3. Psammophilous halophytes. 

4. Shifting or white sand-dunes. 



MacMillan, 1894-1896.— MacMillan's studies of the bogs and muskeags of 
Minnesota constitute the pioneer work upon succession in America, though 



24 GENERAL HISTORICAL SUMMARY 

analysis at this early period was necessarily general. In the investigation of 
Sphagnum atolls (1894:2), he concluded that these atolls, i. e., circular zones 
of Sphagnum, are due to a season of gradual recession of the waters of the 
pond, followed by a season of comparatively rapid increase in area and level. 
This is indicated by the fact that the vegetation of the atoll differs from that 
of the pond outside and the lagoon within it. The atoll first appeared as a 
zone of floating bog, which was separated from the shoreward turf as a conse- 
quence of the original zonation of the shore plants and of the rise of the water- 
level, taken in conjunction with certain special topographic conditions. The 
sequence of events was probably as follows : The pond, as a result of silting-up 
and of climatic variations, slowly diminished until its shore-line coincided with 
the inner edge of the present atoll. The size of the pond at this time is indi- 
cated by the existing lagoon. The shore vegetation then invaded the bare 
slopes and formed characteristic zones, the inner perhaps of Sphagnum. When 
the pond began to fill up again, the marginal zone of turf was forced upward, 
and finally detached to form a circular floating bog or atoll. The further rise 
in level left the atoll well out in the pond. The atoll sank as its weight 
increased with its growth in thickness, and it finally became anchored to the 
bottom of the pond. While it is possible that the two atolls were formed 
simultaneously, one is now in the stage characterized by Sarracenia, Erio- 
phorum, and Kalmia, and the other is dominated by Ledum and Picea. 

MacMillan (1896:500) also studied the Sphagnum moors or muskeags of 
Minnesota, in which almost every stage may be found from open lakes with 
continuous sandy beaches to solid masses of spruce and tamarack. The latter 
is displaced by pines or hardwood, and is finally developed into mixed wood or 
perhaps into meadow. Typical muskeag with spruce and tamarack are 
regarded as an intermediate type between the original open lake and the later 
forest. The center of the muskeag is usually softer than the edges, though in 
many, even of the small ones, the center is quite firmly filled with soil, and 
Sphagnum predominates here. When a central pool is present, it contains 
Utricularia and Lemna, and often Potamogeton and Nymphaea, The next zone 
contains Kalmia and Andromeda, with Carex, Eriophorum, Sarracenia, Salix, 
Vaccinium, etc. Ledum is found on drier peripheral portions, and is often 
the most abundant heath when the Sphagnum has disappeared. This zone is 
surrounded by spruces, usually Picea mariana, sometimes P. canadensis, 
tamarack, Larix laricina, Alnus incana, Betula, and Salix. An examination 
of Sphagnum moors shows that they are characterized by zones of Larix, 
Picea, Ledum, Andromeda, and Utricularia, from the margin to the center. 
The tamarack and spruce zones are slowly closing in upon the others, and will> 
eventually occupy the whole area, as is evidenced by the circular or elliptical 
tamarack communities frequent in southern Minnesota. After the tamarack 
area has become solid, the Sphagnum often persists in little clumps and mats 
at the bases of the trees. Sarracenia, Vaccinium, etc., also linger for some 
time, but Eriophorum, Salix, and many other species disappear because of 
the shade. 

As to the origin of a solid or spruce-centered consocies of tamarack, it is 
doubtful whether a stage with central moor ever existed. In some cases, suc- 
cessions of muskeag openings with intervening tamarack arise from the filling 
of a lake with bars or reefs upon its bottom. Some of the circular tamarack 



RECENT INVESTIGATIONS 25 

swamps with or without spruce cores were not necessarily derived from moors 
with tamarack or tamarack-spruce border-rings, though most of the solid 
tamarack swamps must have developed by the closing in of a ring of timber 
upon a constantly diminishing moor. Finally, the author remarks signifi- 
cantly that "the contemplation of vegetation in any region with these prin- 
ciples in view is certainly interesting. Practically it connects at once ecologic 
distribution with physiography, and enlarges the content both of topography 
and botany. ' ' 

Warming, 1895.— Warming made the first attempt (cf. 1896:350; 1909: 
348) to deal with succession in a general fashion, though his treatment was 
brief and largely incidental to the main purpose of his work. This is empha- 
sized by the fact that the text devoted to this subject is practically unchanged 
in the second edition of his book, in spite of a lapse of 14 years marked by a 
great advance in developmental ecology. Nevertheless, Warming deserves 
great credit for being the first to try to organize this vast field. In the last 
edition the section which deals with development is headed "Struggle between 
plant-communities," and is subdivided into 7 chapters, namely: (1) Condi- 
tions of the Struggle; (2) The Peopling of New Soil; (3) Changes in Vegeta- 
tion Induced by Slow Changes in Soil Fully Occupied by Plants, or Succes- 
sion of Vegetation; (4) Change of Vegetation without Change of Climate or 
Soil; (5) The Weapons of Species; (6) Rare Species; (7) Origin of Species. 
The last two obviously have only a remote connection with succession as a 
process. The discussion of the peopling of new soil deals with the origin of 
bare soil areas and the vegetation which arises upon them. The following 
chapter upon the succession of vegetation treats primarily of water and rock 
seres, and especially of the conversion of moor and forest. The chapter on the 
peopling of new soil is divided into (1) vegetation on sand, (2) production of 
marsh, (3) lowering of water-level, (4) volcanic eruptions, (5) landslips, (6) 
fires in forest and grassland, (7) other sources of new soil, (8) summary of 
results. In the latter, six fundamental principles are laid down ; these deal 
with the pioneers, number of species, life-forms, migration-forms, light rela- 
tions, and the distinction into initial, transitional, and final communities. The 
copious citation of papers on development makes the treatment a very helpful 
introduction to the subject. 

Graebner, 1895. — Graebner (1895 :58) was the first to make a comprehensive 
study of the development of a great climax or subclimax community. The 
developmental relations of the heath of northern Germany are considered in 
three sections: (1) Origin of the Heath Formation; (2) Changes of Heath 
Vegetation; (3) Culture of Heath; while the physical factors are discussed 
under (1) Soils of the Heath ; (2) Dependence of Heath upon Climatic Condi- 
tions; (3) Requirements of Heath Plants. The origin of the heath is dealt 
with under the following heads: (1) Origin of Heath from Forest ; (2) Origin 
of Heath on Bare Sand; (3) Origin of Heath-moor or Moss-moor: (a) in 
water, (&) on bare soil, (c) from forest; (4) Origin of Heath from Heath- 
moor. The details of many of these developmental processes are quoted in 
Chapter VIII. The utilization of the heath is discussed under (1) afforesta- 
tion, (2) cutting of sods, (3) burning, and (4) meadow. 

Pound and Clements, 1898-1900.— Pound and Clements (1898:216; 1900: 
315) also attempted to deal with the origin of formations in a general manner. 



26 GENERAL HISTORICAL SUMMARY 

They distinguished formations as either primitive or recent, with respect to 
origin. By the former was understood the origin in the geological past, while 
recent origin has to do with development at the present time. Formations 
were said to arise at the present time either by nascence or by modification. 
Origin by nascence occurs only upon bare areas, while origin by modification 
occurs through changes in existing communities. Formations regularly dis- 
appear through the agency of fires, floods, man, etc., and in all such cases new 
formations arise by nascence. Two sets of factors are concerned in the origin 
of formations by modification, viz, natural and artificial. Natural factors 
are either biological or physical; artificial factors are due to the presence or 
agency of man or animals. Biotic forces may transform facies or patches into 
formations, or they may change the latter by bringing about the intrusion of 
other facies. Patches ( colonies) are invariably incipient formations, and in 
many situations have become actual formations. 

The physical forces are either meteorologic or physiographic. A rapid 
change from one extreme to another affords the best example of the influence 
of climatic forces. While the instances cited illustrate in a slight degree the 
bearing of climatology upon formations, it is impossible to estimate fully and 
accurately the influence of climatic changes operating through a long period 
or of a sudden reversal of such conditions. Modification of formations by 
physiographic forces is illustrated in the canyons of the Niobrara, where the 
sandy soil has become covered with a layer of loam. Modification due to 
artificial factors is of several sorts. It may arise through the direct agency of 
man, as in the case of culture formations, or through his presence, as in most 
waste formations. The prairie-dog-town waste is an example of a formation 
produced by animal agency. The origin and development of the vegetation 
in blow-outs and sand-draws were described in detail (1898:258; 1900:365). 
The same authors (1898 2 :19) devised the quadrat method for the quantitative 
study of plant communities, and of ecotones especially, and applied it as the 
basic method for determining the structure and development of vegetation. 

Schimper, 1898. — Schimper (1898) has distinguished two ecological groups 
of formations, viz, "climatic or district formations, the character of whose 
vegetation is governed by atmospheric precipitations, and edaphic or local 
formations, whose vegetation is chiefly determined by the nature of the soil. ' ' 
Climatic formations belong to one of three types, forest, grassland, and desert. 
A good forest climate is regarded as consisting of a warm growing-season, a 
continuously moist subsoil, and damp, calm air, especially in winter. A 
climate with dry winters is hostile because trees can not replace the moisture 
lost by transpiration. A good grassland climate consists of frequent, even 
though slight, precipitations during the growing-season, so that the superficial 
soil is kept moist, and a moderate degree of heat as well. Drouth during 
spring or early summer is unfavorable to grassland. A woodland climate 
leads to victory for woodland, a grassland climate to victory for grassland. In 
transition climates, edaphic influences decide the outcome. Strong deviations 
from woodland or grassland climate produce desert. Definite properties of 
the soil may bring forth a character of vegetation that belongs to none of the 
climatic types. These demand a soil congenial to the vast majority of plants. 
Extreme soil conditions unfavorable to most plants set vegetation free from 
the controlling influence of rainfall. Consequently, the vegetation of rocks, 



RECENT INVESTIGATIONS 27 

gravel, swamps, etc., bears in the highest degree the impress of the substratum, 
and this impress usually remains identical under very different climatie 
humidities, which on such soils play only a subordinate part. 

In spite of the successional significance of climatic and edaphic communities, 
Schimper (I. c, 185) seems to have had only a general idea of the development 
of vegetation, for he not only states that little attention had been paid to it, 
but also cites only Treub's study of Krakatoa and the work of Flahault and 
Combres on the Camargue as examples of it. While his open edaphic forma- 
tions are in the main stages in successional development, as he recognizes in 
certain cases, fringing forests are portions of climax and hence climatic forma- 
tions, as is well shown by every large stream of the prairie region. The fact 
that he does not regard edaphic formations as mostly or primarily develop- 
mental is shown by the subdivision into edaphic formations due to telluric 
water (swamps, moors) and open edaphic formations (rocks, dunes). The 
latter alone are regarded as showing a transition from edaphic to climatic 
formations. How close he came to the basic distinction between develop- 
mental and climax communities, and how his concept of edaphic and climatic 
formations caused him to miss the real relation may be gathered from the 
following excerpt: 

"Transition from Edaphic to Climatic Formations: Between the bare hard 
rock and the finely grained soil that finally results from it, for the possession 
of which there is a struggle between woodland and grassland, according to 
what has been said above, there is a series of open transitional formations, 
which possess the character neither of woodland nor of grassland, and which 
assume nearly the same appearance even in dissimilar climates, and owe their 
individuality chiefly to the mechanical texture of the soil. The transforma- 
tion of these transitional formations into the definite ones of woodland and 
grassland is continually proceeding under our eyes, but so slowly that we can 
observe only a part of the process directly, and can form an estimate of their 
sequence only by comparing their condition at different ages. In spite of the 
highly interesting nature of the development of formations, very slight atten- 
tion has hitherto been paid to it. ' ' 

Schimper 's climatic formations are for the most part the climax formations 
of the present treatise, and his edaphic and transition formations are develop- 
mental units, associes, and consocies. This is essentially the conclusion reached 
by Skottsberg, though in different terms (1910:5). 

Cowles, 1899. — The first comprehensive study of succession in America was 
that of Cowles (1899:95) upon the sand-dunes of Lake Michigan. Together 
with the dune studies of Warming already mentioned, it has served as a model 
for the investigation of dune succession the world over. The methods of 
physiography were employed, inasmuch as the flora of a particular area was 
regarded "not as a changeless landscape feature, but rather as a panorama, 
never twice alike." The author concluded that " the ecologist must study 
the order of succession of the plant societies in the development of a region, 
and that he must endeavor to discover the laws which govern the panoramic 
changes. Ecology is, therefore, a study in dynamics." The ecological fac- 
tors of the dunes were considered under the heads: (1) light and heat, (2) 
wind, (3) soil, (4) water, (5) other factors. The plant societies and their 
developmental relations were treated in full under the following captions: 



28 



GENERAL HISTORICAL SUMMARY 



A. The beach. 

1. The lower beach. 

2. The middle beach. 

3. The upper beach. 

4. Fossil beaches. 

B. The embryonic or stationary beach dunes. 

1. Dunes of rapid growth (primary em- 

bryonic dunes). 

2. Dunes of slow growth (secondary em- 

bryonic dunes). 



C. The active or wandering dunes ; the dune 
complex. 

1. Transformation of stationary into 
wandering dunes. 

2. Physical and biological features of the 
dune complex. 

3. Encroachment on preexisting plant 
societies. 

4. Capture of the dune complex by vege- 
tation. 

D. The established dunes. 

1. The basswood-maple series. 

2. The evergreen series. 

3. The oak dunes. 

Cowles, 1901. — Cowles's work (1901:73) upon the physiographic ecology of 
Chicago and vicinity stands out as a landmark in the developmental study of 
vegetation. It forced the recognition of physiography as the most striking 
cause of vegetation changes, and the use of the term "physiographic ecology" 
constantly challenged the attention of students to the attractiveness and sig- 
nificance of successional studies. Cowles deserves great credit at the hands 
of ecologists for his early and consistent championing of the cause of develop- 
ment in vegetation. Even though physiography can not yield a complete 
picture of succession, as Cowles himself recognized (1901:81; 1911:168), its 
processes are so striking and interesting, and its action as an initial cause of 
development so universal and decisive, that it must always receive a large 
share of attention from students of succession. The author's conclusions as 
to progression and regression are considered in detail in Chapter VIII. As a 
consequence, the following outline will suffice to afford a general idea of the 
work, and to indicate its basic nature. 



I. The content and scope of physiographic 
ecology. 
II. The plant societies. 
A. The inland group. 

1. The river series. 

(1) The ravine. 

(2) The river-bluff. 

(3) The flood-plain. 

2. The pond-swamp-prairie series. 

(1) The pond. 

(2) The undrained swamp. 

(3) The prairie. 



II. The plant societies — Continued. 
3. The upland series. 

(1) The rock hill. 

(2) The clay hill. 

(3) The sand hill. 
B. The coastal group. 

1. The lake-bluff series. 

2. The beach-dune-sandhill series. 

(1) The beach. 

(2) The embryonic or stationary 
beach areas. 

(3) The active or wandering dunes; 
the dune complex. 

Summary and conclusion. 

Clements, 1902-1904. — In "Herbaria Formationum Coloradensium" (1902) 
and its continuation, "Cryptogamae Formationum Coloradensium" (1906- 
1908), Clements endeavored to organize an herbarium method of indicating 
and recording the structure and development of vegetation. This method 
was discussed briefly in "Formation and Succession Herbaria" (1904), and 
the analysis of the Colorado vegetation proposed in the collections mentioned 
was sketched in its main details. The formations recognized were largely 
climax associations of the mountain clisere, and were arranged in the corre- 
sponding sequence. Many of them, however, were the developmental asso- 
ciations, now distinguished as associes, and these were grouped in the serai 



RECENT INVESTIGATIONS 



29 



sequence. The structure of each was indicated by the grouping of the species 
into facies, aspects, principal and secondary species, marking consocies, socies, 
clans, and colonies respectively. 

Clements, 1904. — In the "Development and Structure of Vegetation," 
Clements made the first attempt to organize the whole field of present-day 
succession, and to connect the structure of vegetation with its development in 
the essential way that these are related in the individual plant. The concept 
was advanced that vegetation is an entity, whose changes and structures are 
in accord with certain basic principles in much the same fashion that the func- 
tions and structures of plants follow definite laws. The treatment falls into 
five divisions, association, invasion, succession, zonation, and alternation. Of 
these, invasion and succession are developmental processes, and association, 
zonation, and alternation the basic expressions of structure which result from 
them. Invasion was defined as the movement of plants from one area to 
another, and their colonization in the latter. Invasion was analyzed into 
migration, or actual movement into a new place, and ecesis, the establishment 
in the new home. Migration was considered with reference to mobility, 
organs modified for dissemination, migration device, agents, and direction. 
Barriers, endemism, and polygenesis were discussed in connection with ecesis, 
while invasion was further considered with reference to kinds and manner. 
The necessity of using quadrats and migration circles for the exact study of 
invasion was also emphasized. 

After a historical summary of the development of the idea of succession, the 
latter was related to invasion, and successions were classified as normal, 
divided into primary and secondary, and anomalous. Primary and secondary 
successions were grouped upon the basis of agent or process, e. g., elevation, 
volcanic action, weathering (residuary soils), gravity (colluvial soils), water 
(alluvial soils), etc. (c/. Chapter IX). The reactions of serai stages were next 
analyzed in detail, and the laws of succession were grouped under the follow- 
ing heads: (1) causation, (2) reaction, (3) proximity and mobility, (4) ecesis, 
(5) stabilization, (6) general laws. The treatment was concluded by a dis- 
cussion of classification and nomenclature and of methods of investigation. 

Friih and Schroter, 1904. — Although they did not deal specifically with suc- 
cession, the monumental monograph of Friih and Schroter upon the Swiss 
moors is a mine of suceessional material of the first importance. It will 
suffice here to indicate the scope and nature of the work by giving its main 
heads. 



First part: General treatment. 

1. Definitions. 

2. Peat-producing plant formations of 

Switzerland. 

(1) Moor and Peat Communities of the 

Midland, and Jura. 
a. Low moor. 

(a) Deposition and forlanding 

communities. 
(6) Low moor communities. 
o. High moor. 

(2) Moor and peat formation in the 

alpine region. 

3. Peat. 



First part: General treatment — Continued. 

4. Stratigraphy. 

5. Geographical distribution of the Swiss 

moors. 

6. Sketch of a geomorphologic classifica- 

tion of all moors. 

7. Eelation of colonists to moors in the 

light of their toponymy. 

8. Utilization of Swiss moors. 

9. Postglacial vegetation strata of north- 

ern Switzerland, and significance 
of moors in their reconstruction. 
Second part: Description of certain Swiss 
moors. 



30 



GENERAL HISTORICAL SUMMARY 



Clements, 1905-1907. — The treatment in "Development and Structure of 
Vegetation" was adopted in "Research Methods in Ecology," but a further 
attempt was made to place the study of vegetation upon a completely develop- 
mental and quantitative basis. The formation was regarded as a complex 
organism, possessing functions and structures, and passing through a cycle 
of development similar to that of the plant. The formation as a result was 
definitely based upon the habitat as the cause, and a detailed analysis of it 
was made from the standpoint of functions, viz, association, invasion (migra- 
tion and ecesis) and succession (reaction and competition), and of structures, 
zonation and alternation. The formation was analyzed into minor units, 
society, community, and family, for the first time, and the classification and 
nomenclature of units were considered in detail. 

Especial emphasis was placed upon instrumental and quadrat methods of 
exact investigation, in which the constant interaction of habitat, plant, and 
community must furnish the primary basis. Instrumental methods of habitat 
measurement were organized and developed, and the quadrat method of 
analyzing and recording the structure and development of vegetation was 
advanced to the place of first importance in the investigation of succession 
(161). Quadrats were differentiated as list, chart, permanent, denuded, and 
aquatic quadrats of various size, and were modified into line, belt, permanent, 
denuded, and layer transects of varying width and length. A further endeavor 
was made to increase the accuracy and finality of developmental studies by 
organizing an experimental attack upon them, as in "Experimental Evolu- 
tion" (145) and "Experimental Vegetation" (306), by means of methods of 
natural, artificial, and control habitats. Essentially, the same ground was 
covered in "Plant Physiology and Ecology" (1907), though the vegetational 
material was condensed and rearranged, as shown by the following outline : 



X. Methods of studying vegetation. 
XI. The plant formation. 
XII. Aggregation and migration. 



XIII. Competition and ecesis. 

XIV. Invasion and succession. 
XV. Alternation and zonation. 



Moss, 1907-1910. — Moss is entitled to much credit for being the first to 
clearly include the idea of development in the concept of the formation and 
to distinguish formations upon this basis. The importance of his contribu- 
tion in this respect was obscured by an inclusive conception of the habitat, 
which resulted in his restricting the development of the formation to a few 
final stages. However, the germ of the complete developmental view is to be 
found in his distinction of chief and subordinate associations. His views are 
much discussed in Chapters VII and VIII. Hence it will suffice here to point 
out that his concept of the formation was first advanced in 1907 (12), devel- 
oped in 1910, and applied to the vegetation of the Peak district in 1913. 

Clements, 1910. — In the "Life History of Lodgepole Burn Forests," Clem- 
ents endeavored to lay down a set of principles and to furnish a model for the 
exact study of succession by means of instruments and quadrats. Apart 
from the use of the latter, especial emphasis was placed upon the method of 
reconstructing the history of a burned area by means of the annual rings of 
woody plants and perennials, and by means of fire-scars and soil-layers. 
Seed production, distribution, and germination were regarded as the critical 
points of attack, and the consumption of seeds and fruits by rodents and birds 



RECENT INVESTIGATIONS 31 

was held to be of paramount importance. Reaction and competition were 
studied quantitatively for the first time in successional investigation, and 
these were related to the rate of growth and of development. 

Cowles, 1911. — In "The Causes of Vegetative Cycles," Cowles performed 
a distinct service in drawing attention clearly to the three great causes of 
succession, namely, climate, physiography, and biota. While the importance 
of these had been recognized (Pound and Clements, 1898:218; 1900:317; 
Clements, 1904:124), they had not been used for the primary groups in classi- 
fication, nor had their developmental relations been emphasized. While it 
is repeatedly stated in the following chapters that the causal grouping of seres 
is less fundamental and satisfactory than a developmental one, there can be 
no question of its attractiveness and convenience. In fact, it is a necessary 
though not the chief part of a consistently developmental classification. 
Cowles 's ideas are discussed at some length in Chapters VII, VII, and IX, 
and hence only the main topics of his treatment are indicated here. 



1. Demonstration of vegetative cycles. 

2. Development of dynamic plant geog- 

raphy. 

3. Delimitation of successional factors. 



4. Kegional successions. 

5. Topographic successions. 

6. Biotic successions. 

7. Conclusion. 



Shantz, 1911. — In his paper upon "Natural Vegetation as an Indicator," 
Shantz gave the results of the first quantitative study of the reactions and 
successions of a great grassland vegetation. In addition, his studies furnished 
convincing proof of the basic importance of instrumental and quadrat methods 
in investigation, and yielded practical results in a new field of the first conse- 
quence. The study of water penetration, of the relation of root systems to 
it, and of the influence of developing vegetation upon it was a brilliant anal- 
ysis of reaction, and will long serve as a model for all investigators. The 
graphic representation of these relations in a double transect, or "bisect," 
constitutes a new method of record of great value. 

Tansley, 1911.— Tansley and his colleagues, in "Types of British Vegeta- 
tion," were the first to apply the developmental concept to the treatment of 
a great vegetation. Moss's concept of the formation was used in organizing 
the material, and this, combined with a thorough understanding of the basic 
importance of succession, gave to the treatment a distinctively developmental 
character. In this respect, the book is practically unrivaled among accounts 
of extensive vegetations, and its value must always remain great, even if the 
concept of the formation is revised in the light of inci'eased knowledge. Much 
of the work also appeals to the exact ecologist because of the use of instru- 
mental and quadrat methods. Tansley 's views upon the units of vegetation 
are discussed in Chapter VII. 

MacDougal, 1914. — The work of MacDougal and his associates upon the 
Salton Sea is outstanding in several respects. It is unique in dealing with 
xerotropic succession from a wet saline habitat to a climax of desert scrub. 
Still more remarkable has been the opportunity offered by the flooding of the 
Salton Basin and the gradual recession of the lake year by year, thus affording 
a complete record of the stages of development in the series of zones from the 
newest strand of 1913 to the oldest of 1907. It is even more significant, how- 
ever, that the monograph is the result of the cooperation of ten specialists 



32 GENERAL HISTORICAL SUMMARY 

in the various fields represented in this complex problem. This foreshadows 
the future practice of ecology, when the study of vegetation has become so 
largely quantitative that the investigation of the habitat in its climatic, 
edaphic, and physiographic relations must be turned over to the experts in 
these fields. The comprehensive nature of the research is indicated by the 
following outline : geologic history, geographical features, sketch of the geol- 
ogy and soils, chemical composition of the water, variations in composition 
and concentration of water, behavior of micro-organisms, action of Salton Sea 
water on vegetable tissues, tufa deposits, plant ecology and floristics, move- 
ments of vegetation due to submersion and desiccation of land areas. 



III. INITIAL CAUSES. 

Significance of bare areas. — Seres originate only in bare areas or in those 
in which the original population is destroyed. They may be continued, with or 
without change of direction, by less critical modification of the habitat or by 
the invasion of alien species. It is a universal law that all bare places give 
rise to new communities, except those which present the most extreme condi- 
tions of water, temperature, light, or soil. Of such there are few. Even fields 
of ice and snow show algal pioneers, rocks in the driest desert bear lichens, 
caves contain fungi, and all but the saltiest soils permit the entrance of halo- 
phytes. From the standpoint of succession, water is the most important of 
bare habitats, and it is almost never too extreme for plant life, as is shown by 
the invasion of the hot springs of Yellowstone Park by various algae. 

Habitats are (1) originally bare or (2) bare by denudation. The former are 
illustrated by water, land produced by rapid emergence, such as islands, con- 
tinental borders, etc., lava flows and intrusions, deltas, ground moraines, etc., 
dunes, loess, etc. Denuded habitats arise in the most various ways, and are 
best exemplified by bad lands, flooded areas, burns, fallow fields, wastes, etc. 
The essential difference between the two is that the new area is not alone 
developmentally different in never having borne a plant community, but is 
also physically different in lacking the reactions due to successive plant popu- 
lations. The last consideration is of profound importance in the develop- 
ment of the new vegetation, and serves as a primary basis for distinguishing 
successions (plate 2, a, b). 

Modifications of development. — "While a new sere can arise only after the 
destruction of a community in whole or in part, striking changes in the course 
or rate of succession may occur in existing communities. These are only 
modifications of development, and are not to be mistaken for the beginnings 
of new successions. A successional stage may persist beyond the usual period, 
and become a temporary climax, or, more rarely, it may become the actual 
climax. On the other hand, the rate of development may be accelerated, and 
certain normal stages may be combined or omitted. New stages are some- 
times interpolated, or the usual climax may be succeeded by a new climax. 
The direction of development may itself be changed anywhere in its course, 
and may then terminate in the usual climax, or rarely in a new one. These 
are all changes within the succession, and are continuative. They must be 
kept distinct from the destructive changes, which free the habitat for new 
invasions and can alone initiate succession. Developmental modifications are 
produced either by changes in the habitat factors or by changes in the usual 
course of invasion. It is possible also that the two may act together. The 
habitat may be modified in the direction of the successional reaction and corre- 
spondingly hasten the rate of development, or contrary to the reaction and 
thus reduce the rate, fix an earlier climax, or change the direction. In the 
case of invasion it is obvious that the failure of the dominants of a particular 
stage to reach the area would produce striking disturbances in development. 
Likewise, the appearance of alien dominants or potential climax species would 
profoundly affect the usual life-history. 

33 



34 INITIAL CAUSES 

Processes as causes. — In the strictest sense there is perhaps but a single 
universal initial cause of succession, namely, a bare area in which pioneers 
can establish themselves. It is somewhat confusing, if not illogical, to term 
a passive area a cause, and in consequence the term is referred back to the 
active processes and agents which produce the bare area. The latter is the 
initial fact in so far as the development is concerned, but its cause leaves a 
directive result in the form of the physical factors which characterize the new 
area. It must also be recognized that succession does not necessarily occur 
in every bare area. Two other prerequisites must also be met: there must 
be an adjacent or accessible plant population and the physical conditions of 
the habitat must permit ecesis. These are almost universal concomitants of 
bare habitats, the rare exceptions occurring only in the salt-incrusted beds of 
old lakes in arid regions and perhaps in ice-bound polar areas. Further excep- 
tions are naturally furnished by wave or tide swept shores and rocks, but 
these are hardly to be regarded as bare areas. 

Change of conditions. — In a denuded area, moreover, succession proper 
can not occur unless the physical conditions are essentially changed. This is 
especially true when the adjacent population is mobile. In such cases a short 
apparent succession may result, owing to differences in rate of germination 
and growth, but in some cases, at least, the migrants all enter the same year. 
Thus in certain lodgepole pine burns of the Rocky Mountains, firegrass, fire- 
weed, aspen, and lodgepole pine appear together the first year after the fire, 
but there is an apparent sere of three or four stages, due merely to differences 
in rate of growth and consequent dominance. A wholly different example is 
found in certain deserts with one or two distinct rainy seasons, characterized 
by annuals. This is typical of the deserts of Arizona and adjacent parts of 
Mexico and California, in which communities of summer and winter annuals 
appear each season, only to disappear before the subsequent drought. These 
represent the pioneer stage of a succession which can not develop further 
because of extreme conditions. 

A bare area, then, must not merely permit the invasion of an adjacent popu- 
lation ; it must also present conditions that are essentially different if succes- 
sion is to result. This is typically the case, since the conditions of formation 
of new soil differentiate it from the habitats of neighboring communities, while 
the removal of the plant covering materially modifies the habitat, with rare 
exceptions. As a consequence, an initiating process must accomplish two 
results : it must produce a bare area capable of ecesis, and it must furnish it 
with physical factors essentially different, in quantity at least, from the adja- 
cent areas. In short, a bare area, whether new or denuded, to be capable 
of succession must be more extreme than the surrounding habitats. This 
departure from the mean is best seen in the denuding of climax formations, in 
which case the climatic control is disturbed. In the grass formation of central 
Nebraska, denudation by wind erosion produces a departure toward the xero- 
phytic extreme, and by flooding, one toward the hydrophytic extreme. 

Fundamental nature of water-content. — In the vast majority of bare areas 
the departure has to do with water-content, usually its quantity but often its 
quality, as in saline and acid areas. Light is less frequently concerned, while 
changes of other efficient factors — temperature, nutrients, and aeration — 
appear to be subordinate. In all cases the production of a more extreme con- 



CLEMENTS 




PES;- /:**-«•• •*>-. '-.J ."•.*-* Cv' .*-W i 



■,-:--W--SV~- ' «'•'.:,;{'• --Vi**-^ **rWl, r* ■?! k v .vA':>'->:*--'.»*3»J^*».*r»!i.t|.> 




-----233BH 







5 11 



*«* 



%^wi(j&o 



•a~:i 







A. Primary bare area, due to weathering, Mount Garfield, Tike's Peak, Colorado. 
B. Secondary bare area, due to wind erosion, Morainal Valley, Pike's Peak, Colorado. 



INITIAL CAUSES 35 

dition in the new area has two consequences of the first importance. It 
determines the conditions of ecesis and hence the life-forms and species which 
can act as pioneers. It likewise determines the direction of development from 
drier to wetter or wetter to drier, and consequently the reactions possible. 
The degree of departure from the climatic mean controls the life-history and 
determines the number of stages possible between the pioneer and the climax 
vegetation. 

The most critical factor in origin, then, is the amount of water-content in 
comparison with the mean for the climax area. This is directly affected by 
the texture of the soil, and this by the initial process or agent. The two 
extremes possible are water at one end and rock at the other. The former has 
an excess of water-content and a lack of solid material for fixing the habitat ; 
the latter has a surplus of stability and a deficit of water. Between the two 
occur all possible combinations of water and solid materials in the form of the 
various soils. While there is no ecological warrant for excluding rock and 
water from soils, it will perhaps be clearer if the term is restricted to the usual 
meaning of a mixture of comminuted rock and water. Apart from the amount 
of water present in a new area, the stability of the substratum itself must be 
taken into account. This is of the first consequence in extremely mobile soils, 
such as those of dunes and blow-outs, where it determines the form and 
sequence of the pioneers and calls forth a peculiar reaction. The usual course 
of successional development is a response to the increase or decrease of the 
holard, i. e., to the ratio between water and rock, as already suggested. This 
ratio expresses itself in three chief forms, water, rock, and soil. These produce 
primary distinctions in the development of vegetation, and are used as the 
physical basis of the system proposed in Chapter IX. 

Kinds of initial causes. — All initiating processes and agents agree in their 
fundamental relation to succession, viz, the production of a bare area charac- 
terized by a more extreme condition, usually as to the holard. Moreover, 
processes very different in themselves produce areas essentially similar or 
identical as to the sere developed. A pond or lakelet may be formed by 
physiographic processes, such as flooding, filling, or erosion, by a swing of 
climate, by a rise in the water-table, by the action of ice, of gravity as in talus, 
by beavers, or by man in a variety of ways. Many of these do, and all of 
them may, occur in the same climax area, and would then result in identical 
or similar seres. A sandy bank may be formed by currents, waves, ice, wind, 
gravity, or biotic agencies, but the agent has relatively little effect upon the 
succession. It is the wet, loose condition of the bare sand and the surrounding 
vegetation which determine the development. The secondary importance of 
the process is further indicated by the behavior of dune-sand when carried by 
the wind into streams or lakes or heaped into dunes. The water-content of 
the two areas is so controlling that the resulting seres converge only at or near 
the climax. In case base-leveling is regarded as a process, it is obvious that 
here is a process that produces the most diverse bare areas and seres. 

The classification of initial causes from the standpoint of the development 
of vegetation necessarily groups together the most diverse agents and proc- 
esses. This is shown to be the case in the classification of seres outlined later. 
For the sake of a complete account of initial causes it is most convenient to 
treat them here from the standpoint of the nature of the agent or process, 



36 INITIAL CAUSES 

however necessary it may prove to combine them later because of their effect 
suecessionally. In consequence, such causes may be distinguished as (1) 
physiographic, (2) climatic, (3) edaphic, (4) biotic. In the analysis of each 
an attempt is made to distinguish between processes and agents in so far as 
possible. Special attention is given to the results of each in terms of kind of 
bare area and the degree of departure from the holard or other mean. This is 
followed by a discussion of the directive effect upon succession in connection 
with an endeavor to point out the essential nature of each process from the 
standpoint of vegetational development. While every effort has been made to 
appreciate the viewpoints of the physiographer and the climatologist, it is felt 
that these are necessarily subordinate to the main object of analyzing the 
development of vegetation. 

Physiography. — It is necessary at the outset to indicate the scope assigned 
to physiography in the present treatise, since the several definitions of the 
term differ greatly. Physiography is here understood much in the sense used 
by Salisbury (1907:4), who defines it as having "to do primarily with the 
surface of the lithosphere, and the relations of air and water to it. Its field 
is the zone of contact of air and water with land, and of air with water." 
In this definition the emphasis is considered to be upon the phrase "zone of 
contact," and climate is not regarded as covered by the definition. While 
physiography and climate are in constant and universal interrelation, they 
are regarded as coordinate fields. An initial cause is termed physiographic 
when it originates a sere in consequence of a changing land form, as in dunes, 
the cutting down of a lake outlet, or the formation of a delta. It is termed 
climatic when succession results from denudation due to a climatic change 
which critically affects the water or temperature relations of a community. 

Cowles (1911:168) has evidently felt something of the difficulty inhering 
in the various uses of the term physiography, for he contrasts topographic 
with climatic. He apparently also furnishes an example of the double use of 
physiographic. After speaking of biotic changes and climatic changes as 
initial causes of succession, he says: "A third and equally diverse kind of suc- 
cession phenomena was recorded by Reissek in his study of islands in the Dan- 
ube, for here there was clearly recognized the influence of physiographic change 
in vegetation." Here physiographic seems clearly coordinate with climatic 
and biotic, while in the next two sentences it is used to include climatic: 
"Thus, in succession we may distinguish the influence of physiographic and 
biotic agencies. The physiographic agencies have two aspects, namely, regional 
(chiefly climatic) and topographic." Since physiography and topography are 
here regarded as essentially synonymous, it seems desirable for the sake of 
clearness to speak of topographic causes and processes hereafter. 

TOPOGRAPHIC CAUSES. 

Topographic processes. — All the forces which mold land surfaces have one 
of two effects. They may add to the land or take away from it. The same 
topographic agent may do both, as when a stream erodes in its upper course 
and deposits a delta at its mouth, or undercuts one shore and forms a mud- 
bank or sand-bank along the other. In similar fashion, a glacier may scoop 
out a pond or a lake in one region and deposit the material as a moraine 



TOPOGRAPHIC CAUSES 37 

in another. The wind may sweep sand from a shore or blow-out and heap 
it up elsewhere, or it may carry dust from dry lake-beds or flood-plains for 
long distances and pile it in great masses of loess. Gravity in conjunction 
with weathering removes the faces of cliffs and accumulates the coarse mate- 
rial in talus slopes at the base. 

Volcanoes and ground-waters in the form of hot springs and mineral springs 
act similarly to the extent that material is taken from one place and added 
to another. They differ from the agents cited above, however, in that the 
removal is from the interior of the earth 's crust as a rule, and bare areas are 
consequently produced only by addition. Perhaps the formation of sink- 
holes may well be regarded as an exception, where the collapse of the surface 
results directly or indirectly in denudation. Volcanoes change land forms 
principally by means of lava-flows and deposits of volcanic dust, and mineral 
springs by deposition of dissolved material as travertine, sinter, etc. In the 
case of weathering, the process itself neither adds nor subtracts, but is so 
intimately and universally associated with transportive agents — water, wind, 
ice, and gravity — that the effect is the same. Residuary soils furnish the 
only example of weathering without transport, but these are of little impor- 
tance in succession. 

Kinds of processes. — The various processes which control land forms, and 
hence the surface available for succession, are (1) erosion, (2) deposit, (3) 
flooding, (4) drainage, (5) elevation, and (6) subsidence. From the stand- 
point of physiography, it is evident that these are more or less related in pairs 
of complementary processes. Erosion in the upper part of a valley has its 
inevitable effect in the deposition which characterizes the lower part. The 
formation of a lake by flooding has its normal outcome in drainage by the cut- 
ting down of the stream which flows from it, unless filling or evaporation 
proceed too rapidly. Elevation and subsidence are theoretically complement- 
ary at least, and on the Scandinavian coast it is assumed that they are asso- 
ciated at the present time. As will be shown later, elevation and subsidence 
have practically no effect upon succession at the present, except in the rare 
cases where new land suddenly appears. Moreover, grave doubt has been 
thrown upon many of the supposed evidences of coastal changes of level. 

While erosion and deposit, flooding and drainage are complementary in the 
life-history of a river system, as processes they are opposite or antagonistic. 
The clue to their influence upon vegetation is not to be found in the fact that 
they are associated in the base-leveling of a region. It resides, on the Contrary, 
in the fact that one is destructive of vegetation or habitat and the other con- 
structive as to habitat. In general, erosion lays bare or destroys an existing 
habitat, deposition produces a new one. Flooding destroys an existing habi- 
tat and drainage lays bare a new one. The fact that all produce bare areas 
upon which successions can arise is no evidence of their relationship from the 
standpoint of vegetation. Bare habitats are also produced by climate, fire, 
man, or animals, without indicating any essential relationship among them. 
Viewed as topographic processes merely, the sharp contrast between erosion 
and deposition is obvious. Indeed, in this respect, they are exact opposites. 
Erosion removes the surface of a land form or decreases its area, or it may 
do both in the same case. Deposit adds to the surface, or increases the area 
of the land form, or both. Their union in the development of a river system 



38 INITIAL CAUSES 

has furnished a basic and fertile viewpoint for physiography, but it seems to 
possess no such value for vegetation. 

Base-leveling. — The complex topographic development of a region known 
as base-leveling seems to present a fundamental explanation of those seres 
initiated by topographic changes. But the relation between base-leveling and 
the development of vegetation is apparent rather than real. The connection 
between them appears to be incidental but not fundamental. There is no such 
correspondence between the life-history of the Mississippi system and its 
vegetation as an intrinsic relation between the two would demand. The serai 
development from origin to climax is a wholly different thing in northern Min- 
nesota from that found in Louisiana, in spite of similarly swampy habitats, 
and must always remain so while the present climatic relations persist. This 
seems even truer of mature streams which flow northward, such as the Mac- 
kenzie, in which the upper and lower courses must develop in the midst of 
very different climax formations. In the case of the great drainage basin of 
the Mississippi, differences in climate and climax vegetation make the course 
of succession very different in areas of the same age topographically. On the 
other hand, the valley of the Platte is much more mature than that of the Nio- 
brara or Running Water, but both streams flow through the same climax 
formations with the same developmental history. 

Similar evidence is afforded by lakes and flood-plains developed at different 
stages in the life-history of a river. According to Davis (1887), a young drain- 
age system contains many lakes which disappear by filling and draining as the 
river matures. New lakes may then form by the damming back of tributaries, 
by the cutting off of meanders to form ox-bow lakes, and by the production of 
lakes in the delta. At any time in the course of the development, lakes may 
also arise by accidents, such as lava-flows, ice, landslips, work of man, etc. In 
the same climax region the succession in all these lakes will be essentially iden- 
tical, regardless of their relation to the life-history of the river. It can be 
changed only by a decisive change in climate which produces a new climax 
formation. In the prairie region the succession in cut-off lakes of mature 
rivers duplicates in all essentials the development in lakes belonging to the 
youth of tributary streams. 

The one striking connection between base-leveling and succession seems to 
lie in the fact that bare areas for colonization are naturally most abundant 
when erosion and deposition are most active. Since erosion is typical of hills 
and deposition of valleys, bare areas produced by erosion tend to be dries 
than the mean, and those produced by deposition to be wetter. In consequence, 
just as hill and lowland tend to reach a mean in a temporary base-level, so 
vegetation tends to a mean, which is usually mesophytic. That it is the 
extremes and the climatic mean which control, however, and not the topo- 
graphic process, is shown in semiarid and desert regions. In the Santa Cata- 
lina and Santa Rita ranges of Arizona the torrential rains cut back deep 
canyons and carry out the detritus in enormous alluvial fans known as 
bajadas. The vegetation of the bajada, instead of being more mesophytic 
than that of the forested slopes or the moist upper canyons, is intensely 
xerophytic. A similar condition exists in the Uncompahgre Plateau of Colo- 
rado, where the extensive table-land is covered with spruce and fir forest with 
a rainfall of 30 inches or more, while the streams carry eroded material away 
into an Artemisia-Atriplex vegetation with a rainfall of 12 inches. 



EROSION 39 

If we consider wind erosion and deposit instead of that by water, it seems 
to afford the clue to the puzzle. While wind erosion is of much less impor- 
tance, it still plays a large part, as seen in the hundreds of thousands of square 
miles covered by dunes, sand-hills, and loess deposits. Here the process is 
totally opposed to base-leveling, as the sand or dust is blown from strand or 
plains into dunes or hills. The significant fact is that the hills and crests are 
driest, the hollows wettest. Controlled by water-content extremes, seres of 
totally different initial stages arise in these two areas, converge more and 
more as they develop, and terminate in the same climax. In consequence the 
actual explanation appears to lie in the fact that in the usual erosion by water, 
soil and water move together. The water which falls on a hill leaves the crest 
or slope with the soil it has eroded away. When it reaches the ravine, stream, 
or lowland it deposits its load, only to be itself entrapped in large degree. 
Thus, it is evident that topography, with soil texture, is the great middleman 
distributing rainfall to the various habitats as water-content. It is this rela- 
tion which one finds repeated again and again in a drainage basin, in youth, 
in maturity, and in old age, wherever erosion and deposition occur. The age 
of the basin seems to affect the relation only in so far as it determines the 
number or steepness of slopes on which erosion can occur, or the area of low- 
land where deposits can accumulate. 

EROSION. 

Nature. — The removal of soil or rock by the wearing away of the land 
surface is erosion. In the case of rock it is often preceded by weathering, 
but the process consists essentially of corrosion, the picking up of the loose or 
loosened material and of its transportation. Weathering is too universal and 
too well understood to warrant discussion here. In so far as plants play a 
part in it, it will be considered under "Reactions" (p. 83). A distinction 
between corrasion and transport is difficult if not impossible. With wind and 
water, the picking up of weathered particles involves carrying them as well, 
while gravity transports or affects transportation without picking the material 
up. In the beds of streams or glaciers, however, corrasion plays an essential 
part in freeing material for transport. Where some part of the rock is dis- 
solved in the water, in the process of corrosion, the distinction from transport 
is also very slight. 

As a rule the agent that picks up the material is the one which transports 
it, as is evident in the erosion of a gully or the scooping out of a sand-hill or 
dune. Often, however, material freed by gravity, as in talus slopes, is trans- 
ported by water or wind. The distance of transport varies within the widest 
limits. In residuary sdils the conversion of the rock takes place by weather- 
ing alone. Loosened material may be carried a few millimeters into the cracks 
of rocks, or it may be carried hundreds of miles and into totally different habi- 
tats and regions. The distance of transport naturally determines the place of 
deposit, but it will suffice to consider the latter alone. 

Agents of erosion. — The great agents of erosion at the present time are 
water and wind. The action of ice, while of paramount importance during the 
glacial period, especially in transport, is now limited and local. The effect 
of gravity, combined with weathering, is less extensive than that of wind and 
water, but the areas so produced are of great service in studying succession, 



40 INITIAL CAUSES 

owing to their number and relatively small size. Of topographic agencies, 
volcanoes alone produce no erosion, unless the violent removal of portions of 
volcanic cones be regarded as such. 

In erosion, agents usually act alone, though it is often the case that one 
agent will erode an area deposited by another. It is true that water and 
gravity are regularly associated in erosion by water, but gravity is hardly 
to be regarded as controlling, except in the disintegration of peaks and cliffs, 
and in the case of avalanches, whether of snow or of rock and soil. 

Rate and degree of erosion. — While the force and duration of the chief 
eroding agents, water and wind, differ greatly, they are critical in determin- 
ing the rate of erosion and the degree to which it will act. These are also 
affected in the first degree by the hardness and compactness of the surface 
acted upon, as is shown by the formation of boulders and ledges in rock strata. 
The erosive force of rain-water depends upon the rate of precipitation and the 
angle of slope, that of running water upon the fall or current and the load 
carried. While these vary in all possible degrees, the essential fact is that they 
are more or less constant for a particular area. In many areas they are sus- 
ceptible of approximate measurement and expression, at least. The erosive 
force of wind is determined by the velocity and by the exposure of the slope 
acted upon. Prairies and plains, deserts, ridges, mountain peaks, and shores 
are the chief areas characterized by forceful winds. Apart from velocity and 
exposure, the erosive influence of wind is determined by the length of the 
period for which it acts and the frequency of such periods. Certain areas, 
sand-hills, dunes, strands, and mountain-tops, for example, may have winds 
forceful enough to pick up sand or dust, every day for all or most of the year. 
In the case of compact soils or rock surfaces the action of the wind is confined 
to removing weathered material, unless the wind carries a load of abrasive 
particles. 

In the case of water erosion, intensity often compensates for lack of dura- 
tion or frequency, especially where the slope is great and vegetation scanty. 
This is especially true of regions with torrential rains, such as the deserts of 
the Southwest and the Black Hills and Rocky Mountains, where the charac- 
teristic "bad lands" occur. The density or hardness of the eroded surface, 
its roughness, and the amount and kind of dead or living cover, together with 
slope and exposure, are all factors of moment in determining the final effect 
of erosion. These are factors which permit of quantitive study with a minute- 
ness and thoroughness not yet attempted. Such .study seems inevitable if we 
are to make an accurate analysis of the forces which influence migration and 
occupation, and direct the water-content basis of successional development. 

Fragmentary and superficial erosion. — Erosion may act over the whole 
surface of an area with greater or less uniformity; it may be restricted to 
particular portions or localized in the most minute way. Striking illustration 
of this is found in the comparison of ridge and slope with valley. Moreover, 
while the contrast between slope and. valley is of the greatest, similar slopes 
exhibit similar or identical behavior. Marked examples of local erosion by 
wind are found in the blow-outs of sand-hills and dunes, while sand-draws 
and washes furnish similar cases of water erosion. Fragmentary erosion is a 
feature, however, of lateral erosion by running water, and of cliff and ridge 
erosion due to gravity. It furnishes a bewildering array of areas of all sizes 



EROSION 41 

and degrees, which present wide conditions for ecesis. For this reason it 
offers material of the first importance for reconstructing the course of succes- 
sion and relating the various stages. Superficial erosion to varying depths is 
likewise a ready source of developmental clues. When produced artificially 
under control, both processes furnish an invaluable experimental method of 
studying succession by denuded quadrats and transects (plate 3 a). 

Bare areas due to water erosion. — The important areas laid bare through 
erosion by water are: (1) gullies, ravines, and valleys; (2) sand-draws; (3) 
washes; (4) flood-plains and river islands; (5) banks; (6) lake-shores and 
sea-shores; (7) crests and slopes; (8) bad lands; (9) buttes; (10) monad- 
nocks. In some of these, such as stream-banks, the erosion is chiefly or wholly 
lateral, and hence more or less local and fragmentary. In others, e. g., washes 
and flood-plains, the erosion is superficial and general, and is often intimately 
associated with deposition. The majority of them are the result of inter- 
action of both methods, as illustrated in the production of a gully or ravine 
or a sea-shore. Bad lands and beaches represent, perhaps, the extreme condi- 
tions of erosion, in which colonization is all but impossible. In all, the success 
of initial invasion depends upon the kind of surface laid bare and the water- 
content as determined by the surface, the slope, and the climatic region. The 
form and nature of the area itself are important only as they affect these 
factors. 

Bare areas due to wind erosion. — The most characteristic areas of this 
sort are wind-denuded areas of dunes and sand-hills, particularly the well- 
known blow-outs. Related to these are the strands from which the dune- 
sand is gathered by the wind, and the plains of rivers, lakes, and glacial 
margins from which sand-hills and loess deposits have been formed by wind 
action. Wind is a powerful factor in the erosion of strands, but at the 
present it is of slight importance in flood-plains and lacustrine plains as 
compared with its action in Tertiary and Quaternary times. The abrasion 
and removal of material from exposed peaks, ridges, and slopes of rocks is 
constantly going on, but it does not often assume such striking proportions 
as are seen in the characteristic mushroom rocks found in the Rocky Moun- 
tains. It plays some part, and often a controlling one, in the lichen and moss 
stages of the rock succession (plates 1a, 2 b). 

Bare areas due to gravity. — Many areas owe their origin to the action of 
gravity on material freed by weathering, or in some cases by water erosion. 
In the case of mountains, relatively large areas are exposed by exfoliation, 
crumbling, or slipping. In certain mountain regions with heavy snowfall, the 
effect of gravity on the snow-fields produces numerous characteristic snow- 
slides in which the ground is often swept bare. Crumbling and slipping are 
also universal processes on the steep slopes of crests and hills, and along 
stream-banks and lake and sea shores everywhere. From their hardness, 
instability, or dryness, and the steep or vertical faces, such areas are among 
the slowest to be invaded as a rule. In consequence, they often permit the 
persistence of initial stages or their recurrence long after they have disap- 
peared elsewhere (plate 3 b). 

Bare areas due to ice action. — At the present time, the effect of ice in pro- 
ducing bare habitats is confined to wind-exposed shores and to the margins 
of glaciers. In the latter case the final condition of the area is naturally due 



42 INITIAL CAUSES 

in large degree to fluvial action as well. During the glacial period erosion of 
the hardest rocks or of softer materials to great depths was the universal 
accompaniment of glacial movement. In the Rocky Mountains and Sierra 
Nevada the extreme conditions which rock invaders must meet are often the 
direct outcome of glacial scouring in the past. 

The action of wind-driven ice on exposed shores is a striking feature of 
many lakes in Minnesota and Wisconsin as well as elsewhere. Shores other- 
wise similar are differentiated by the grinding and pushing action of the ice. 
Bare shores are modified in various ways, while those covered with vegeta- 
tion are denuded more or less completely (plate 3 c). 

DEPOSIT. 

Significance. — Deposit is such a regular and often such an immediate con- 
sequence of erosion that it is desirable to emphasize the fact that this essen- 
tial relation, which is so fundamental to physiography, is of little or no 
consequence in the development of vegetation. Material eroded in one part 
of a drainage basin must in the usual course be deposited in another part, 
and in both cases it bears a direct relation to the development of the river 
system. This would be in no wise true of the development of the vegetation 
in the two areas, especially if the latter were in different climatic regions. 
Even in the same climatic region it is true only of final or subfinal stages. 
This latter fact, however, indicates no essential relationship, since all initial 
causes in the region give rise to seres which reach the same climax. It must 
also be recalled that the great deposits of marl, peat, travertine, sinter, and 
volcanic dust bear no relation to a preceding erosion. 

The relation of deposit to the future development of vegetation depends 
upon a number of factors. These are: (1) the agency of transport; (2) kind 
of material; (3) manner of deposition; (4) rate, depth, and extent; (5) place 
and distance of deposit. These determine the rate at which the sere can 
develop, the physical conditions which the invaders must meet, the climax 
vegetation from which they can be drawn, and the effect of migration. 

Agents of deposit. — If the term is used in the inclusive sense, the agents 
of deposit are : (1) running water ; (2) ground-water; (3) wind; (4) glaciers, 
ice, and snow; (5) volcanoes; (6) gravity. Plants and animals also build 
deposits, but these are naturally considered under biotic agencies and under 
reactions. Just as it is practically impossible to draw a line between the 
loosening of material and its transport, so it is often equally impossible to 
separate transport from sedimentation. In any area of deposition the two 
are going on simultaneously, the dropping of part of the load carried by 
water, for example, permitting the further transport of the remainder. Deltas 
and alluvial fans are especially fine examples of the sorting due to the inter- 
action of these two processes. They make it clear that any unit deposit is 
due to the varying distances of transport of the particles, as well as to the 
fact of their fall. However, in the case of a single particle, it is evident that 
this is first transported and then deposited, after which it may be transported 
and deposited again and again. In the study of a sediment actually forming, 
the last phase of transport must be included in deposition. 

As is true of erosion also, two agents may interact in effecting deposition. 
The ordinary relation between two agents is successive, as in the case of 



CLEMENTS 






V,. -I 




A. Superficial erosion by water on clay hills. La Jolla, California. 
B. Bare areas due to the action of gravity, Canon of the Yellowstone River, 

Yellowstone 1'ark. 
C. Bare areas due to the action of ice, Yoseniite Valley. California. 



DEPOSIT 43 

beach-sand thrown up by the waves and finally deposited as dunes by the 
wind, or in the probable wind formation of loess from water-laid plains. In 
many cases, however, the action of the two agents is more or less simultane- 
ous. This is especially true of the fluvio-glacial deposits due to the combined 
action of water and glaciers, and of beaches formed by the action of wave- 
borne ice. It is peculiarly characteristic of the deposits formed by ground- 
waters in surface streams, though here we are really dealing with a single 
agent, as is essentially true also in the case of snow-drifts due to wind. As to 
volcanoes, eruptive activity is the one agent concerned in lava-flows and 
cinder-cones, but this is combined with wind to effect the transport and 
deposit of volcanic dust. 

Manner of deposit. — This depends upon the kind and nature of the agent 
and upon the kind of material. Ground-waters carry material in the finest 
condition, since it is in solution, and hence such deposits as sinter and traver- 
tine are the most uniform of all in composition and texture, if certain char- 
acteristic irregularities of surface are disregarded. Such deposits owe their 
uniformity and density, moreover, to the fact that the water contains cement- 
ing material alone, so to speak, while in the case of surface-water the solid, 
particles are in much larger quantity than the material in solution. Winds 
also carry particles of a small range of size, and the resulting deposits are 
essentially homogeneous. As a consequence of the lack of cementing material 
in solution, dunes, sand-hills, masses of volcanic dust, etc., are also character- 
istically unstable. An exception to this is furnished by loess, though the 
stability here is perhaps due to the later cementing action of absorbed water. 

Water and ice exhibit the widest range in the size of the materials carried 
and in the amount of cementing action present. This is of course particularly 
true of glaciers. They show the most striking difference in the sorting of 
materials, moreover, as is well known. Lateral sorting is practically absent 
from true glacial deposits, while it is typical of water sediments. Glacial 
deposits possess much less cohesion in consequence of this fact and of the 
wider range in the size of particles, but also because of the greater lack of 
cementing substances. 

The nature of the solid particles and of the cementing materials is also 
a determining factor in the hardness of the deposit. While an uncemented 
deposit is ready for invasion as soon as water conditions warrant, sedimentary 
rock must first be weathered before it will permit penetration or possess the 
requisite water-content. Eocks cemented by lime respond most readily to 
weathering processes, though many exceptions are produced by differences in 
the amount of cement, quite apart from its natui'e, and also by pressure and 
metamorphism. Differences in the material of the particles, as between 
sand and clay for example, are controlling as to the holard and echard, and 
are consequently decisive in the ecesis of pioneer migrants. 

Rate and depth of deposit. — The rapidity with which a deposit accumu- 
lates depends upon the amount of material carried, upon the duration or 
frequency of the agency, and upon the barrier to movement which effects 
the deposition of the load. The rate of deposit is of importance in determin- 
ing the rate at which vegetation is overwhelmed and at which the deposit 
will reach a point where colonization will be possible. It also affects the 
reactions of the early stages of succession, as well as the period of each. These 



. 



44 INITIAL CAUSES 

are often more directly related to the continuous or intermittent nature of 
the deposition than to the rate itself. The depth of the deposit is chiefly an 
effect of the rate and duration, but it also has to do with the area as well, a 
fact axiomatic of lowlands (plate 4 a). 

Place of deposit. — The place of deposit is critical for two reasons: (1) it 
controls the water conditions of the new area, and (2) it determines the 
climatic area and the climax formation in which the new sere will develop. 
Places of deposit fall into two distinct groups, namely, (1) in water, (2) on 
land (plate 4). These differ primarily, and sometimes only with respect to 
the extremeness of conditions as to colonization. Deposits in water must be 
built up to a level at which submerged plants can ecize before the sere proper 
begins, a process which is often a matter of centuries and ages. They can be 
invaded only by water-plants, and the early stages of succession are often 
very long. Deposits on land, however, can be invaded at once. The physical 
conditions are necessarily further from the extreme, a wider range of life- 
forms can enter as pioneers, and the stages of development are usually fewer 
and shorter. 

Deposit by water is regularly in water, except in such cases as surface wash, 
but the withdrawal of flood-waters produces what is essentially a deposit on 
land. Aeolian deposits, on the contrary, are mostly on land, primarily because 
the material composing them is picked up from beaches and flood-plains by 
winds blowing from the water area. In the case of dunes, however, they may 
be carried into lakes, ponds, and swamps, and initiate a sere widely divergent 
from that on the dunes proper. The course of successional development also 
depends upon deposition in salt water, fresh water, or alkaline water. Water 
deposits may be changed into land areas by drainage and elevation, and the 
land deposits into water deposits in vegetational effect by flooding and subsi- 
dence. The elevation of water deposits has naturally been a chief initiating 
cause of the great eoseres of the geological past. Gravity deposits occur with 
equal readiness and in countless numbers along sea-coasts, lake-shores, and 
stream-banks, and in all hilly and mountainous regions. Along shores they 
are in land or water or both ; in the case of hills and mountains they are typi- 
cally land deposits. Glacial deposits produce both land and water areas, 
though they are first actually laid down in water as the ice melts. The same 
is true of fluvio-glacial deposits, though these necessarily show more relation- 
ship to water deposits. 

Distance of transport. — Transported material is deposited at every con- 
ceivable distance from the place of origin. It may be washed by water or 
blown by wind into a crack a few millimeters distant, or it may be carried 
thousands of miles and find its resting-place on the bottom of the ocean. 
Water deposits may be found at the greatest distance from their source, and 
glacial deposits come next in this respect. The range of wind deposits at the 
present day is much less, while deposits due to volcanoes, ground water, and 
gravity are local. Distance naturally effects no sharp distinction between 
deposits, but it is a factor to be considered, especially in the relation of the 
new area to migration and to climax vegetation. From this point of view it is 
profitable to distinguish (1) deposition in the minor community where the 
material originated, (2) in other communities of the same climax association, 
(3) in areas controlled by earlier stages of a sere, but in the same climatic 



DEPOSIT 45 

region, (4) in another climatic region, and hence another climax formation. 
As is at once evident, the point of initiation, the course of development, and 
the final climax all hinge upon the effect of distance. 

Fragmentary and local deposit. — As has just been seen, deposits may be 
local, as well as of extremely small area. The clearest examples of this are to 
be found in weathered rocks, cliffs, and ledges, where deposit occurs in tiny 
cracks, in clefts, or in large fissures. Here the deposit is often so slight that 
the plants growing in it seem to be growing on rock, and hence to belong to 
the initial stage of the rock sere. A careful scrutiny shows that they are not 
true rock-plants, comparable with lichens and mosses, but that they are soil- 
plants, or in some cases water-plants. Local deposit in small separate areas, 
like local erosion, produces innumerable small communities, each with its 
proper place in the sere, but often so surrounded or interrupted by plants of 
other stages that great confusion results (plate 4 a). In much work that has 
been done upon succession so far, the course of development, the movement 
of the population, and the relationship to the physical factors have been lost 
or confused by the failure to recognize how detailed and accurate this scru- 
tiny must be. As is shown later in full, only the use of exact quadrat and 
transect methods can show the way in such cases. 

Sterility of deposits. — Deposits vary greatly in the numbers of dissem- 
inules found in them, a factor of considerable importance in the development 
of the first stages. The number of viable propagules depends upon the 
source of the material as well as upon the agent. The deposits of wind-borne 
volcanic dust and of sinter and travertine formed by ground-water represent 
the one extreme of almost absolute sterility. Primary dune-sand, i. e., blown 
more or less directly from the beach, probably comes next, while secondary 
dune-sand from established dunes would contain more seeds and fruits. 
Glacial deposits are sterile, though terminal and lateral moraines of existing 
glaciers are relatively an exception. Water deposits contain disseminules in 
varying numbers, but for the most part they are relatively rich, though the 
viability of many of the seeds is usually low. Talus deposits, land-slides, etc., 
tend to contain the maximum number of seeds and fruits, owing to the fact 
that plants and plant parts are so often carried down with the falling mate- 
rial, and to the favorable conditions for the preservation of seeds in a viable 
condition. 

Bare areas due to deposit by moving water.— Under this term are 
included (1) streams and run-off and (2) waves, tides, and shore-currents. 
The typical areas of deposit by running water, which includes streams of all 
degrees as well as surface run-off, are the following: (1) alluvial cones, fans, 
bajadas, etc.; (2) alluvial plains; (3) flood-plains; (4) channel deposits; 
(5) deltas; (6) beds of lakes. Topographically, the first three are much 
more closely related in the essentials of the process of formation than their 
names indicate. It is practically impossible to distinguish between an alluvial 
plain and a flood-plain, if they are not indeed identical. Alluvial cones and 
fans often merge into a complex, which is called by Salisbury (1907:183) a 
piedmont alluvial plain. It is clear that the sand-bars of a river differ in 
little but form from the deltas made in it by lateral streams, and in the case 
of a braided river such as the Platte, the different streams of the network 
may form deltas, lateral banks, and median bars in the same channel. More- 



46 INITIAL CAUSES 

over, deposition in the bed of a stream is very similar to that in the bed of 
a lake, a similarity that becomes identity when a stream is ponded anywhere 
in its course. Finally, alluvial fans and deltas are very like in both form and 
development. The delta of many a mountain stream is different in no essen- 
tial lea cure from the alluvial cones produced by surface wash or by temporary 
rivulets along its course. It seems evident, therefore, that the above six 
forms due to water deposit present no inherent topographic differences cap- 
able of controlling successional development. As initial causes they are 
practically identical, and it is necessary to turn to the materials and water 
relations of the new soil to discern the real factors (plate 4). 

Bare areas due to waves and tides. — These are (1) beaches, and (2) reefs, 
bars, and spits. Beaches are produced by the daily interaction of erosion and 
deposit, but their soil is chiefly a result of deposit. They are peculiar to lakes 
and seas in name only, for many rivers possess shores identical with them in 
formation and structure. Bars, reefs, and spits are merely different forms 
resulting from the same process, and, when adjacent, exhibit the same devel- 
opment of vegetation. Such differences as exist are the result of variations 
in height, composition, kind and amount of water, etc., and are often found 
in different parts of the same spit or bar (plate 4 c). 

Composition and water-content of alluvial deposits. — The material of 
recent or existing water-laid deposits may consist of (1) silt, (2) clay, (3) 
sand, (4) gravel, (5) rubble or shingle, (6) boulders, or (7) marl. It is 
sorted in such a way as to be essentially homogeneous in any one place, 
though it may vary much between distal and proximal areas. The various 
materials differ chiefly in the size of the particles, and through the latter 
influence the course of succession. The size of particles affects the water- 
content, usually in decisive fashion, and it also determines the cohesion and 
resulting stability. It also has some relation to the solutes present, though 
these are dependent upon the kind of material as well as the source of the 
water. Here, as elsewhere, water exercises the controlling influence in direct- 
ing development, either by its amount or its quality. As to the latter, it may 
be (1) fresh, (2) saline, (3) alkaline, (4) acid. When it is other than fresh, 
the early stages of the sere are characterized by a decreasing extremeness 
and a final return to fresh water as water-content, after which the amount of 
the holard is decisive (plate 4 b). 

Bare areas due to deposit by ground-waters. — Characteristic deposits are 
made by mineral springs, especially hot springs, and geysers. While deposi- 
tion occurs usually about or near the spring or opening, it is also frequent in 
the resulting streams, and may even occur in ponds or lakes at some distance 
from the source. Such deposits consist of (1) travertine or tufa, (2) siliceous 
sinter or geyserite, and (3) salt. Travertine is formed from waters highly 
charged with lime, and is deposited in lakes of dry regions, as well as from 
spring-waters and their streams. In a large number of cases its formation is 
due to algae, but it also arises directly from chemical solution. Sinter or 
geyserite is typical of the areas about geysers, where it arises by deposition 
from the hot siliceous waters, through the action of algse. It also results from 
the decomposition of siliceous minerals about the fumaroles of volcanic 
regions. Both travertine and sinter are rocks and exhibit the general relation 
of rocks to succession. Their first colonists are algae and lichens, which 



CLEMENTS 




A. Local ami fragmentary deposit in a young fayine, bad lands, Scott's Bluff, Nebraska. 

1?. Sand-bars due to deposit in streams, North Platte River, Scott's Bluff, Nebraska. 

C. Silting up of the Soledad Estuary. La Jolla, California. 



DEPOSIT 47 

slowly weather the surface and collect organic material for later stages. Salt 
may be deposited from spring-waters, as in salt basins, or by the water of 
lakes in arid regions where evaporation exceeds the inflow. In moist and 
semi-arid regions the salt crust is usually thin, and hence readily dissolved or 
weathered away, permitting halophytes to enter and begin the succession. In 
arid regions, on the contrary, the deposits are thicker, and removal by 
weathering or solution is nearly impossible, so that extensive areas in Utah 
and Nevada remain absolutely sterile under present conditions. 

Bare areas due to deposit by wind. — The principal wind deposits are (1) 
sand, chiefly in the form of dunes; (2) loess; (3) volcanic dust, Of these, 
dunes, both inland and coastal, are much the most important at the present 
time. Loess, while covering enormous areas in the valleys of the Mississippi, 
Rhine, Danube, Hoang-Ho, and other rivers, is not in process of formation 
to-day, and the prisere developed upon it can not now be traced in the actual 
course of development. Deposits of volcanic dust are infrequent and localized, 
and cover relatively small areas. They are unique in the suddenness and 
completeness with which the area is covered and in their absolute sterility. 

Dunes are classical examples of deposits which initiate succession. Their 
wide distribution and striking mobility have made them favorite subjects of 
investigation by both physiographer and botanist, and there is probably no 
other initial area and succession of which we know so much. In spite of their 
characteristic topography, however, dunes affect succession by virtue of insta- 
bility and water relations, and not by form. This is shown by the inland 
dunes or sand-hills of the Great Plains. Hills, deep hollows or blow-outs, and 
sandy plains show the same development, regardless of their differences of 
form. In all of these the controlling part is played by the sand-catching and 
sand-binding plants, usually grasses, which act as pioneers. The chief reac- 
tions are three, namely, fixation of the sand, gradual accumulation of humus, 
and decrease of evaporation and increase of holard (plate 1a). 

Dune plants have often been regarded as halophytes, but since Kearney 
(1904) has shown that this is rarely true of strand species, it seems impos- 
sible to distinguish initial dune areas on the basis of salinity. This is borne 
out by the similarity of the early stages of shore dunes, whether lacustrine or 
marine. As a result of their location these often differ much in the later 
stages, and especially in the climax. Inland dunes occur in widely different 
climatic regions and differ from each other in population as well as from 
coastal dunes. This is well illustrated by the sand-hills of Nebraska, the 
"white sands" of southern New Mexico, and the barehans of Turkestan. 

Deposit by ice and snow. — Of these agencies, glaciers have been of much 
the greatest importance in the past, though their action to-day is localized in 
mountains and the polar regions. The effect of shore-ice, though interesting, 
is rarely sufficient to produce a distinct result. The influence of snow is often 
striking and decisive, but it is also peculiar to mountain regions. Naturally, 
all of these show a close dependence upon water, as is seen in the water 
relations of the resulting soils. 

Bare areas due to deposit by glaciers. — From the standpoint of succession 
there is no essential difference between glacial and fluvio-glaeial deposits. 
This is readily explained by the fact that glacial materials are really deposited 
in water at the time of general melting. The effect upon the new soil is prac- 



48 INITIAL CAUSES 

tically the same as when it is water-laid after being carried for some time 
from the glacier. In the case of drumlins, indeed, it seems probable that they 
may be due either to fluvio-glacial deposits or to erosion by an ice-sheet of an 
antecedent ground-moraine. Hence it seems immaterial whether the deposit 
is glacial, e. g., lateral and terminal moraines, ground-moraines, or drumlins, 
or fluvio-glacial, such as valley trains, outwash plains, eskars, kames, or 
drumlins (plate 5 a). 

The essential effects of glacial deposition are produced by the size and uni- 
formity of the particles and by the place of deposit, i. e., on land or in water. 
While fluvio-glacial deposits often show more sorting, glacial soils proper show 
all possible variations. A till sheet may consist of gravel, sand, or clay, but 
frequently of all three. It may contain pebbles, or boulders, or the deposit 
may be largely made up of enormous blocks. The latter present the extreme 
conditions for rock succession, while the till sheet proper offers an area pre- 
pared for a higher type of colonists. The ratio of sand or gravel to clay deter- 
mines the holard and echard of the till and the invasions upon it. This is 
relatively immaterial when deposit occurs in water, but is significant in the 
ordinary case of deposit on land, particularly where there is considerable 
initial relief. Here the influence of slope upon water from the melting ice is 
the same as upon ordinary precipitation. The ridges are drier, valleys wetter, 
and the slopes intermediate, and the course of succession varies accordingly. 

Bare areas due to deposit by ice and snow. — The action of shore-ice is a 
combination of erosion and deposit, though when a shore-wall is thus formed 
it is a true deposit. Its structure, depth, and extent usually distinguish it 
but slightly from the ordinary shore. In consequence, the development of 
vegetation upon it rarely produces any distinctive features. 

Deposit in consequence of snow action is confined to snow-slide masses and 
to flat areas or hollows in which snow melts. In such snow-hollows the 
deposit is usually insignificant, but the accumulation of the dust and sand 
brought to the snow-field by wind often becomes appreciable in a few years. 
In practically all cases the real effect is produced by the partial destruction 
of vegetation by the snow and the ponding of the snow-water. 

Snow-slides may be assigned either to snow or gravity, since they are due 
to the combined action of both. They are more frequent than land-slides of 
like extent, but they differ from them in few respects. A snow-slide sweeps 
away the vegetation more or less completely, but may disturb the soil to a 
slight depth only. A heavy fall of snow may initiate a land-slide, however. 
The mass of detritus at the bottom of a snow-slide is much more homogeneous 
and contains more plant material than do most land-slides. It differs also in 
the fact that it may require one or more summers for the snow to melt. During 
this time the mass remains cold and wet, and invasion is correspondingly slow. 

Bare areas due to deposit by gravity. — Talus masses and slopes are uni- 
versal deposits at the base of cliffs, shores, banks, etc. From the nature of 
their formation they differ chiefly in composition, apart from differences due 
to location. The initial conditions presented are often very like those of the 
cliff or bank above. The chief change is one of density or coherence. This 
is well shown by the fact that the lichens and cleft plants of a granite cliff or 
wall are usually found in the talus as well, even when this shows the degree 
of disintegration found in a gravel-slide. Eock talus, in consequence, really 



CLEMENTS 




A. Tormina] moraine of the Nisqually Glacier, Mount Rainier, Washington; l>a 

due to deposit by a glacier. 
B. Talus slopes of Scott's Hluff, Nebraska; bare areas duo to gravity. 






DEPOSIT 49 

continues the pioneer stage begun on the fragmenting area. The develop- 
ment is hastened by the more rapid weathering and the greater irregularity 
of surface, which permit corresponding variations of holard. Talus derived 
from soils such as sand or clay, or from rocks which decompose readily, 
presents typically more extreme conditions as to water-content and stability 
than the fragmenting area. The initial stages upon talus are hence new 
stages, and show much less relationship to the population of the top of cliff 
or bank (plate 5 b). 

The location of the talus is important in determining its water relations, as 
well as its possible population. Along banks that are being undercut, the 
material is swept away, but when the current leaves the shore the talus is 
often built up in the water. This happens not infrequently on lake-shores 
as well. In both cases the excessive holard which results initiates the succes- 
sion with a hydrophytic or amphibious stage. When the talus accumulates 
on land, as is the rule, the initial holard is typically less than the normal for 
the particular soil in the climatic region concerned. This arises from the 
looseness and unevenness of the talus and the corresponding ease of evapora- 
tion from the soil. In desert regions this tendency becomes decisive, and the 
colonization of the south and west slopes is extremely difficult. 

Bare areas due to volcanic deposits. — Volcanic agencies bring about 
deposits of lava, of cinders so-called, of ash or dust, and of sinter. Deposits 
of ash have already been considered briefly under ' ' Wind. ' ' The local deposi- 
tion of ash is less influenced by wind, and the depth of accumulation is often 
very great, sometimes reaching 50 to 100 feet. On the cones themselves it is 
frequently much greater. Coarser material — cinders, rocks, and enormous 
stones — are also blown from craters in great quantities and fall near the cone 
or upon its slopes. The lava and mud expelled from volcanoes flow in streams 
from the crater. Eivers of lava have been known to reach a length of 50 miles 
and a width of half a mile. In flat places the stream spreads out and forms 
a lava lake which hardens into a plain. Mud volcanoes are small, geyser-like 
structures which discharge mud. They build up small cones, which are 
usually grouped and cover considerable areas with their deposits. The 
deposits due to volcanoes or geysers regularly result in the destruction of 
vegetation, but this. effect may be produced alone, as a consequence of the 
emission of poisonous gases, steam, hot water, or hot mud, of fire-blasts or 
the heating of the soil. Such bare areas are characteristic features of 
Yellowstone Park. 

All volcanic deposits are characterized by great sterility. They are usually 
small in extent, and hence easily accessible to migrants. The ease of invasion 
depends largely upon the coherence of the deposit. Invasion takes place 
more readily in ash and cinders than upon lava, unless they are quite deep. 
Mud deposits would apparently be invaded most readily. The seres of vol- 
canic deposits have been little studied, but it is known that they are relatively 
long. As would be expected, this is particularly true of lava, though climate 
exerts a decisive effect, as is shown by the invasion of lava fields in Iceland 
and Java. 

Ponding and draining. — These constitute a second pair of related but 
opposite topographic processes. Flooding or ponding is almost inevitably 
followed by draining, and the drainage of an area may be obstructed and 

5 



50 INITIAL CAUSES 

result in flooding at almost any time. Ponding usually produces extreme 
water conditions, with a corresponding effect upon succession. Drainage 
reduces the depth of water, or extreme holard, and accordingly shortens the 
development. Although opposite as processes, the two may produce exactly 
the same initial area. This is probably a frequent occurrence. Ponding may 
be so shallow as to permit the immediate entrance of hydrophytes or of 
amphibious plants. Drainage may reduce the level of pond or lake without 
completely emptying it, and thus produce similar depths for invaders. Here 
again it is evident that the water relation of the new area is decisive, and not 
the originating process. 

Kinds of lakes and ponds. — Lakes and ponds have been classified from 
many points of view, such as form, origin, physiographic development, chem- 
ical nature, climatic region, depth, duration, etc. All of these bases have 
some relation to succession, though this seems least direct in the case of 
physiographic development and manner of origin. The amount and kind of 
water are the controlling factors in determining when the pioneer stages of 
a sere can develop, just as the extent and location determine the pioneers and 
the rate at which they can invade. Depth, as modified by evaporation, filling, 
and draining, is the critical point upon which invasion turns. Depth, extent, 
and kind of water are unimportant points to the geographer, however, and his 
classification can not be expected to reveal basic vegetational values. It 
does, however, bring out many points which the ecologist must note in con- 
nection with water and soil relations. The classification proposed by Eussell 
(1895) perhaps serves this purpose best because of its detail. The agencies 
of lake and pond formation are grouped as follows: 



1. 

2. 


New land depressions. 
Atmospheric. 

(1) Wind rock basins. 

(2) Dune ponds. 


5. 


Volcanic. 

(1) Damming by lava. 

(2) Damming by ash. 

(3) Crater lakes. 




3 


Aqueous. 


6. 


Meteoric? 






(1) Streams. 


7. 


Earthquakes. 






(a) Excavation and change of bed. 


8. 


Organic agencies. 






(6) Lateral deltas and valley cones. 




(1) Coral? 






(c) Ponding of tributaries. 




(2) Peat moss. 






(d) Ox-bows or cut-offs. 




(3) Beaver dams. 






(e) Delta lakes. 




(4) Eiver rafts. 






(2) Waves and currents. 




(5) Gas mud -holes. 






(a) Bar lakes. 




(6) Wallows. 




4 


Glacial. 


9. 


Diastrophic. 






(1) Damming of laterals by ice. 


10. 


Land-slides. 






(2) Damming by drift. 


11. 


Chemical action. 






(3) Scouring. 


12. 


Interaction of two or more 


agents 



Life-history of a lake. — While the relation of a lake to its river system 
seems to have no significance for succession, it is evident that the life-history 
of a lake shows a direct relation. A lake in a humid region matures by filling 
with detritus, by the cutting down of its outlet, by the shallowing action of 
plants, or by the combined influence of these processes. When the depth is 
reduced to a few meters, pioneer hydrophytes appear. From this point 
the maturity and depth of the lake and the succession of the vegetation are 
but different aspects of one complex process. At any time in this process the 
lake may be rejuvenated by an increase of the water-supply, by the damming 
of the outlet, or by the sinking of the basin. All of these have the same effect, 



DEPOSIT 51 

namely, that of increasing the depth of the water. The vegetation is wholly 
destroyed if the depth increases more than a few feet in the early stages of 
the sere, or even a few inches in later grassland and woodland stages. A new 
sere of the same succession is initiated as soon as the water is again shallowed 
to a point where submerged hydrophytes can ecize. The significant fact is 
that the development of the sere in the original and in the rejuvenated lake 
will be essentially or wholly identical. Physiographically, the two lakes are. 
essentially different. As initial areas for succession, they are identical. 

In the case of a lake in an arid region, evaporation is the chief factor in 
shallowing, though filling by detritus plays a part. The cutting down of the 
outlet is of little or no importance, owing to the reduction in volume. The 
shallowing effect of vegetation can be felt only in relative youth, as the 
increasing salinity destroys the plant population. Hence, the development of 
the usual water sere ceases long before the death of the lake, in such bodies of 
water as Great Salt Lake for example. Consequently there is no correspond- 
ence between the life-history of the lake and the development of the vegeta- 
tion. Instead of a drying lake-bed, densely clothed with plants, the salt- 
incrusted bottom is entirely devoid of vegetation. In an arid climate it is 
only after many years that the salt crust is sufficiently destroyed by solution 
and removal to permit the appearance of pioneer halophytes. If a striking 
increase of rainfall or the accession of new streams should rejuvenate the 
lake, the initiation of a sere would depend largely upon the freshness of the 
water. If vegetation does appear its development will be determined by 
whether the water remains fresh or is renderd saline by evaporation. 

In the case of periodic ponds, playa lakes, etc., the drainage is usually such 
as to prevent the accumulation of solutes. The life of the pond is often short, 
and hydrophytic vegetation may fail to develop altogether, the sere beginning 
on the exposed, rapidly drying bottom. Ponds which last for several years 
permit more complete expression of the water sere. Salt ponds supplied by 
springs, such as the Salt Basin at Lincoln, are unique in behavior. They pos- 
sess no phanerogamic vegetation at all, except when greatly diluted by fresh- 
water streams. Ordinarily they dry up, forming a salt crust, as in the lakes 
of arid regions. The periodic ponds of the Great Plains also deposit alkali 
when they evaporate, though this is often not so great in amount as to prevent 
the invasion of halophytes. The latter initiate a short succession which ter- 
minates in the climatic grassland. 

Drainage. — Drainage by cutting down the outlet of the lake or pond plays 
some part in shallowing most lakes of humid regions. It is usually subordi- 
nate to filling, and in effect is indistinguishable from it. As an initial cause 
of new areas for succession, it is most evident where natural barriers which 
produce ponding are suddenly removed, and especially where it is resorted to 
intentionally by man. The effect of drainage upon the course of development 
is determined by the degree to which the water is removed. If open water 
with a depth not greater than 12 meters is left, the normal water succession 
is initiated. Later stages are initial at the respective depths less than this, 
until a point is reached where drainage completely removes the surface water. 
This permits the soil to dry more or less rapidly and to become quickly cov- 
ered with a growth of mesophytic ruderals or subruderals. This is typically 
the case in areas drained artificially. 



52 INITIAL CAUSES 

The effect of drainage is perhaps most striking when the level of the lake is 
reduced to a point where islands and peninsulas appear. The rate of lower- 
ing determines the water conditions at which invasion becomes possible, and 
hence the presence and length of hydrophytic stages. In the behavior of a 
river-plain after flooding, draining differentiates the area into ponds and 
mud-flats, in which the water sere will appear in a variety of stages. 

ELEVATION AND SUBSIDENCE. 

Elevation and subsidence. — These are likewise examples of processes 
which are opposite and complementary in many instances. This is seen in the 
relation of syncline and anticline, in earthquakes, in the movements of cer- 
tain sea-coasts, etc. Naturally they affect the development of the vegetation 
directly only where land and water are in contact. The relative rise or fall 
of an inland area could produce no effect upon succession, except as it changed 
the climate or produced flooding or draining. A direct effect upon vegetation 
is possible only along sea-coasts and lake-coasts, and of course upon islands. 
Here, however, the rate of emergence or submergence is the decisive factor. 
Physiographically, it is much the same thing whether a coast rises or falls a 
foot a year or in a thousand years. From the standpoint of succession, the 
rates of elevation and subsidence assumed for the Atlantic coast, the coast of 
Sweden, and for the Great Lakes are so slow as to be wholly insignificant. A 
rise of 2.5 feet in a century is equivalent to a rise of less than a centimeter 
per year. Over such a minute area as would result from such a rise each 
year, there would be no chance for extreme conditions and no place for even 
the most incomplete and fragmentary succession. Each annual increment of 
space would be controlled by the association at hand and quickly made an 
intrinsic part of it. 

With subsidence the case is different if it results in flooding and consequent 
destruction of the vegetation. In the case of a low-lying coastal forest even 
the slow rate of 3 feet per century would eventually flood the forest floor and 
kill the trees. It is conceivable that the flooded forest floor might serve as a 
new area for the invasion of a coastal swamp, thus causing an apparent 
"retrogressive" succession. It seems much more probable, however, that the 
action of waves and tides would erode the soil, and thus destroy the forest or 
other vegetation piecemeal year after year. 

While elevation and subsidence are largely negligible to-day as initial 
causes of succession, this is obviously not true of the past. Crustal movements 
were of the first importance in changing the outlines of continents, in building 
mountains, and in producing cycles of erosion. As a consequence of such 
changes they exerted a profound effect upon climate. The consequent effect 
upon vegetation resulted in the change of climax populations and in the 
initiation of a new eosere. 

New areas due to elevation. — As assumed above, elevation produces new 
areas for succession only where it is relatively rapid, or where the new area is 
not in contact with an existing vegetation. Cases of this kind are practically 
confined at the present day to volcanic cones formed in lakes or in the ocean, 
to islands due to volcanic disturbances, and to coral reefs and islands. The 
latter, however, belong properly under biotic initial causes, though their 
formation and behavior are often intimately associated with volcanic islands. 



ELEVATION AND SUBSIDENCE 53 

Apparently no studies have been made as yet of the development of vegeta- 
tion on new islands due to volcanic action. It seems evident, however, that 
they would exhibit rock and cinder seres generally, with the water sere of 
coral and other oceanic islands at the lowest level. 

Subsidence. — The evidences for the recent subsidence of the Atlantic coast 
of the United States are summed up by Johnson (1913:451), as: 

(1) Wholly fictitious appearance of changes of level; (2) phenomena pro- 
duced by local changes in tidal heights without any real change in the general 
level of either land or sea; (3) phenomena really produced by a sinking of the 
land, but so long ago that they can not properly be cited as proofs of subsi- 
dence within the last few thousand years. The fictitious appearance of change 
of level is given by (1) standing forests killed by the invasion of the sea; (2) 
submerged stumps; (3) submerged peat. Lyall, Cook, Gesner, Ganong, and 
others have regarded dead standing forests as conclusive evidence of the sub- 
sidence of the Atlantic coast. Goldthwaite found that death resulted in soma 
cases from fire and in others from a local rise in the high-tide level. Johnson 
ascribed three distinct causes for the death of the forests about Cascumpeque 
Harbor in Prince Edward Island. Accumulations of sand in the forest caused 
the ponding back of storm-waters, and the consequent death of the trees. 
Elsewhere, small waves had eroded the earth from the tree-roots and exposed 
them to salt water. Finally, the number and width of the inlets in the barrier 
beach had caused a local rise in the high-tide surface and a consequent inva- 
sion of the forest by salt water. The author explains the presence of live 
cypresses in water often over 5 feet deep by finding that the spreading bases 
were just above water-level at the same elevation as on the adjacent low shore. 
and that the submerged parts were really spreading roots. The trees had 
grown on a low coast composed of peaty soil, and the erosion of the latter by 
the waves had left the trees standing in water. 

Submerged stumps are found to arise in a variety of ways independently of 
coastal subsidence. Along the shores of South Carolina and Georgia, small 
waves undermine the trees and let them down into salt water, often in the 
erect position. The trunks later break off at the water-line and leave upright 
submerged stumps. Similar stumps are also produced by the long tap-roots 
of certain trees, such as the loblolly pine. Submerged stumps, due to a local 
rise of the high-tide level, to the compression of peat-bogs caused by a lower- 
ing of the ground-water level as the waves cut into the shoreward side of such 
bogs, to the compression of deposits due to the weight of barrier beaches, as 
well as to many other causes, have been observed. It is also pointed out thai 
beds of submerged peat containing stumps may be caused by the sinking of 
floating bogs, by the lowering of the ground-water and the consequent lower- 
ing of the surface of the bog when the latter is encroached iipon by the sen 
or by the weight of a barrier beach, compressing the peat so that it is exposed 
at or below tide-level on the seaward side of the beach. 

The phenomena produced by a local rise in the high-tide level are explained 
in detail. A bay nearly separated from the sea by a barrier beach, but con- 
nected with it by a narrow tidal inlet, will have a lower high-tide level. Trees 
and other vegetation will grow down to the high-tide level of the bay. and 
hence below the high-tide level of the sea. Whenever a large breach is made 
in the barrier beach the high-tide level will become the same in the bay as for 
the sea. All trees whose bases are below this high-tide level will be killed 
and will later be represented by submerged stumps. The surface of the salt 
marsh will build up to the new high-tide level, enveloping stumps and other 
plant remains. Fresh-water peat may also be buried under a layer of salt- 



54 INITIAL CAUSES 

marsh peat. Changes of high-tide level with the killing of forests have 
actually been observed, as near Boston in 1898, when a large opening was 
made in a barrier beach by a storm. 

Appearances of subsidence predominate over those of elevation because 
marsh deposits tend to sink to the new level when the high-tide level is 
lowered because the immediate destruction of fresh-water vegetation by salt- 
water when the high-tide limit is raised is more striking than the slow 
recovery of marine areas by fresh vegetation when the high-tide level ia 
lowered, and because in the cycle of shore-line development retrograding 
exceeds pro-grading, and retrograding tends to carry higher tide-levels into 
low lands, where apparent changes of level are most easily recognized. 

The above conclusions have been given in some detail because there can be 
no question of the existence of fictitious evidences of subsidence. On the 
other hand, it is equally clear that existing subsidence would produce similar 
or identical phenomena, as Davis (1910:635) has well shown in the case of 
the layers of salt-marsh grasses. To the ecologist the actual facts of coastal 
seres and coseres are the important ones, upon which must be based the final 
decision as to the causal action of subsidence or of change of tide-level. It 
is clear, however, that slow subsidence, whether recent or remote, can only 
destroy amphibious or land vegetation and preserve the plants more or less 
completely in the form of peat as an evidence of former communities. It 
can not initiate a new area for colonization, except in so far as the ocean 
itself may be so regarded. 

Earthquakes. — Practically no attention has been paid to the effects of 
earthquakes in producing new areas. It is obvious that a variety of different 
areas may arise from the action of earthquakes, either directly or indirectly. 
The direct effects are seen in the emergence of land from water and the sub- 
sidence of sea-coasts and deltas, and in the formation of small craters and 
mud-cones. Indirect effects arise in valleys where the drainage is disturbed 
by faults or otherwise, and new areas are consequently formed by ponding 
or draining. Earthquakes also loosen masses of rock or soil from cliffs and 
slopes, producing talus and slide masses. The great tidal-waves of earthquakes 
must also produce striking effects in denudation, erosion, and ponding on 
coast lands and islands in their path. An earthquake is thus a primary cause 
which has erosion, deposit, flooding, draining, elevation, and subsidence at 
its command in producing bare areas on which succession will occur. 

Similarity of topographic processes. — The paired character of topographic 
processes has already been remarked. Erosion and deposit, flooding and 
draining, elevation and subsidence are all pairs of opposite and more or less 
related processes from the standpoint of topography (plates 2 a, 7 b). To 
the ecologist, however, they are alike in being initial processes which produce 
new or denuded areas for succession. From this viewpoint their similarity 
depends upon the water relations of the new area. Flooding and subsidence 
produce new water areas, draining and elevation new land areas. Gradual 
deposition in water makes the latter susceptible to colonization, while erosion 
exposes the land surfaces to invasion. Theoretically, at least, it is possible 
for all of these six processes to produce bare areas of essentially the same 
water-content within the same climax region, and hence to initiate the same 
succession. The shores of large lakes do actually exhibit the same water 
succession in initial areas produced by each of the four processes, deposit, 



EDAPHIC CAUSES— CLIMATIC CAUSES 55 

flooding, draining, and erosion. It would be altogether unusual for elevation 
and subsidence to be added to these, but it could at least happen in a region 
subject to earthquakes or volcanic disturbances. It must have occurred 
repeatedly in geological periods characterized by great diastrophic changes. 

EDAPHIC CAUSES. 
Nature.— In the preceding account of topographic processes which pro- 
duce bare areas, it has frequently been shown that the critical results are the 
soil structure and the amount and kind of water-content. This is equally true 
of new areas due to climatic and biotic agents. In the case of all initial 
causes, therefore, the basic control is exerted by water-content, which is con- 
trolled in its turn by the physical character of the soil. A change in the kind 
of soil-water may seem an instance of an edaphic initial cause. The real 
cause, however, is topographic in the case of a change from salt to fresh water 
or the reverse, climatic in the increasing alkalinity of the lakes and pools of 
arid regions, and biotic when acid or other injurious substances accumulate 
in the soil. All seres are consequently more or less edaphic in nature, and 
hence the term edaphic can not well be used to distinguish one kind from 
another or to contrast with climatic causes. If seres are grouped in accord- 
ance with initial causes, they can be distinguished only on the basis of the 
forces which lie behind the changed conditions of soil and water. These are 
topographic, climatic, and biotic agents. In a developmental classification of 
seres such a basis is believed to be of secondary value. While such a group- 
ing is simple and convenient, it is artificial because it ignores development, 
and because of the fact that very dissimilar initial causes produce identical 
bare areas and seres. 

CLIMATIC CAUSES. 

Bole. — Climate may produce new areas for succession, or it may modify 
existing seres by changing the rate or direction of development, by displacing 
the climax, etc. As a cause of modification, the discussion of climate belongs 
elsewhere, and will be found in the chapters on direction, climax, and eosere. 
We are concerned with it here only as an initiator of new seres. In this role 
it acts usually through the agency of the ordinary changes and phenomena 
which constitute weather. A distinction between climate and weather is 
manifestly impossible. It is clear that climate would produce less effect in 
the course of its ordinary oscillations than when it swings beyond the usual 
extremes. A change of climate can produce bare areas by direct action only 
when the change is sudden. A slow departure, even if permanent, would act 
upon existing vegetation only by modifying it through ecesis or adaptation. 
Indirectly, of course, climatic factors and processes may cause new areas 
through the cooperation of topographic or biotic agents. 

Bare areas due to climatic factors directly. — The direct action of climatic 
factors takes place regularly through the destruction of existing vegetation. 
When the destruction is complete or nearly so, a bare area with more extreme 
water conditions is the result. The factors which act in this manner are: 
(1) drouth, (2) wind, (3) snow, (4) hail, (5) frost, and (6) lightning. In 
addition, evaporation, which is the essential process in drouth, produces new 
areas from water bodies in semiarid and arid regions. It may have the same 
effect on periodic ponds in humid regions. While the process is the same. 



56 INITIAL CAUSES 

the degree to which it acts varies widely. Evaporation may merely reduce 
the water-level to a point where the ecesis of hydrophytes is possible, or it 
may continue to a point where islands, peninsulas, or wide strips of shore 
are laid bare to invasion. Finally, the lake or pond may disappear entirely, 
leaving a marsh, a moist or dry plain, or a salt crust. 

Bare areas due to drouth. — The action of drouth in destroying vegetation 
and producing areas for colonization is largely confined to semiarid and arid 
regions. In humid regions it is neither frequent nor critical, while in desert 
regions it is the climax condition to which vegetation has adapted itself fully 
or nearly so. The usual effect is to produce a change in existing vegetation, 
but in regions like the Great Plains it sometimes destroys vegetation com- 
pletely. As a rule, the destruction operates upon cultivated fields, simply 
freeing the area somewhat earlier for the usual development of a ruderal 
stage. It also occurs occasionally in tree plantations, with somewhat similar 
results. In native vegetation the complete destruction of a community is 
rare. When it does occur it is nearly always in lowland communities which 
have followed streams far beyond their climatic region. Ruderal and sub- 
ruderal communities which pioneer in disturbed soils are the most frequent 
sufferers. In desert regions, which are characterized by communities of 
summer and winter annuals, the destruction of the latter by drouth before 
the vegetative season is over must occur occasionally. It has no significance 
for succession, however, as it is wholly periodic. 

Bare areas due to wind. — The direct action of the wind upon vegetation is 
seen only in so-called "wind-throws" in forests. While areas in which trees 
have been blown down by the wind are frequent in some regions, they are 
local and of small extent. They are most apt to occur in pure stands of such 
trees as balsam, spruce, and lodgepole pine. " Wind- throws " are frequent in 
mountain regions where the soil is moist and shallow. The action of the wind 
affects only the tree layer, in addition to tearing up the soil as a consequence 
of uprooting the trees. It is supplemented by evaporation, which destroys 
the shade species by augmenting their transpiration greatly at a time when 
the holard is being constantly diminished by the drying out of the soil. As a 
consequence, "wind-throws" may become completely denuded of vegetation. 
In the case of completely closed forests, such as mature forests of the lodge- 
pole pine (Pinus murrayana) and Engelmann spruce (Picea engelmanni) , 
the fall of the trees amounts to denudation, since occasional saprophytes are 
often the only flowering plants left. 

Bare areas due to snow, hail, and frost. — Bare areas due largely to snow 
are restricted to alpine and polar regions, where they occur usually in a zone 
between the area always covered with snow and that in which the snow dis- 
appears each summer. An abnormal fall or unusual drifting will cause the 
snow to remain in places regularly exposed each summer. After a winter of 
less precipitation or a summer of greater heat, the drifts or fields will melt, 
leaving a bare area for invasion. This frequently happens in the denser 
portions of coniferous forests, as well as in and around the outposts above the 
timberline. In such cases the resulting development has to do chiefly with 
the undergrowth. 

The effect of frost in producing bare areas by destroying the plant popula- 
tion is almost negligible. Its action is confined almost wholly to cultivated 



CLIMATIC CAUSES 57 

areas, such as orchards, fields, and gardens. In such places only the first 
pioneers of a ruderal population can appear, except in rare cases where the 
area is abandoned because of the frost. Communities of ruderal annuals are 
sometimes destroyed by frost, but this delays the usual course of succession 
for but a year at most. Native vegetation may be changed by the action of 
frost, but can rarely be wholly destroyed by it, because of the persistence of 
perennial species with underground parts. A single case of the destruction 
of native communities by frost was found in the dune areas of Medanos Spit, 
near San Diego, in southern California. The severe freeze of 1912 had com- 
pletely killed many large families of Mesembryantkemum, and these still per- 
sisted as blackened areas in the spring of 1914. Such areas had been essen- 
tially denuded by frost, and were already being invaded by other pioneers. 

The denuding action of hail is often very great. In some parts of the Great 
Plains destructive hailstorms are so frequent that they have caused the aban- 
donment of farms and sometimes of whole districts. As with frost, the effect 
upon cultivated plants is very much greater than upon native vegetation. It 
is not infrequent to see the fields so razed by hail that not a single plant is 
left alive. Native communities often suffer great damage, especially broad- 
leaved forests and scrub, but the effect rarely approaches denudation. Grass- 
land is sometimes mowed down also, but the effect is merely to favor the 
grasses at the expense of species with broad leaves or rigid stems. 

Bare areas due to lightning.— The role of lightning in causing fire in vege- 
tation has come to be recognized as very important (Bell, 1897 ; Clements, 1910 ; 
Graves, 1910; Harper, 1912). The majority of lightning strokes do not set 
fire to trees or other plants, and the attendant rain usually stops incipient 
burns. Even in such cases forest fires have actually been seen to start from 
lightning, and the number of such cases in the aggregate would apparently 
be large. In regions with frequent dry thunderstorms, i. e., those unaccom- 
panied by rain, such as occur especially in Montana and Idaho, lightning is 
the cause of numerous, often very destructive, fires. Once well started there 
is no difference in a forest fire due to lightning and one due to other agents, 
such as man, volcanic eruptions, etc. 

Bare areas due indirectly to climatic factors.— These are due almost 
wholly to the effect of physiography in exceptional cases of rainfall, of run- 
off due to melting snow, or of wind-driven waters. In all three the process is 
essentially the same. The normal drainage of the area is overtaxed. The 
flood-waters reach higher levels than usual and are ponded back into depres- 
sions rarely reached. Moreover, they cover the lowlands for a much longer 
period. In the one case they form new water areas for invasion. Since these 
are usually shallow and subject to evaporation, the development in them is 
a short one. In the case of the lowlands, the vegetation of many areas is 
washed away, covered with silt, or killed by the water, and the area is bared 
for a new development. This is of course essentially what must have occurred 
at the end of each period of glaciation. The ponding back of glacial waters 
and the fluvio-glacial deposits were the outcome of the interaction of climate 
and physiography, just as can be seen in miniature at the foot of a glacier 
to-day. 

Sudden changes of climate. — It is probable that there is no such thing as 
a sudden change of climate, apart from the striking deviations from the 



58 INITIAL CAUSES 

normal that we are so familiar with. If the criteria of evolution and of histor- 
ical geology are applied to climatology, it seems evident that even the climates 
of the past are largely to be explained in terms of present climatic processes 
(Huntington, 1914). If we consider the causes which are thought to produce 
the most striking and sudden deviations at present, namely, sun-spot maxima- 
minima and volcanic ash in the atmosphere, two facts are evident. The first 
is that the period between extremes is several years. Whatever the effect 
may be in sorting out the population, or in producing adaptation, it is clear 
that the intensities known, when spread over several years, are quite insuffi- 
cient to destroy plant communities and thus denude habitats. The second 
fact is that there is no record of the destruction of vegetation at such periods,- 
though doubtless the effects of frosts were then most marked. In consequence 
it seems impossible to regard changes of climate as initial causes of succession. 
They are effective only in modifying existing seres. 

BIOTIC CAUSES. 

General relations. — In considering the influence of animals and plants 
upon succession, it is necessary at the outset to distinguish clearly between 
biotic causes and biotic reactions. The former, like all initial causes, produce 
bare areas on which a new sere can develop. Biotic reactions, on the con- 
trary, have nothing to do with the production of initial areas, but represent 
the modifying action of each stage upon the habitat. They are continuative, 
since they induce and control the successive waves of invasion which mark 
the various stages. A plant or animal parasite which produces a bare area by 
killing all the plants of a community, as may readily occur in families or pure 
stands of trees, is a biotic initial cause. Holophytes and saprophytes can only 
react upon the habitat by changing the factors of air or soil. Earthworms 
react upon the soil conditions, while rodents such as prairie-dogs both react 
and initiate new areas. It is the reactions of the plant communities upon 
the habitat which are of paramount importance. "With the possible exception 
of Sphagnum, plants very rarely play the role of initial causes. The reverse is 
true of man and animals. They are initial causes of great frequence and wide- 
spread distribution, but only a few have a definite reaction upon the habitat. 

Like climatic factors, biotic agents may change the existing vegetation, as 
well as initiate new vegetation. In both cases they have to do with develop- 
ment, but they can be regarded as causes of succession only when they pro- 
duce bare areas in which invasion occurs. It is probable that animals change 
the course of development more often than they start it, while the activities 
of man lead largely to denudation. 

Action and effect. — Man, and animals to a certain extent also, have at 
their command the initial processes already considered under topography. 
These are removal, deposit, drainage, and flooding. In addition, they may 
destroy the vegetation, but affect the soil slightly or not at all. In the case 
of man, in particular, the most various activities result in similar processes 
and areas. It seems most natural to group them accordingly, rather than to 
consider them from the standpoint of the activities themselves. This is illus- 
trated by the fact that fallow fields, roadsides, prairie-dog towns, and ant- 
hills in the prairie region exhibit essentially the same condition and initiate 
similar or identical developments. The most suggestive grouping in conse- 



BIOTIC CAUSES 59 

quence is the following: (1) activities that destroy vegetation without greatly 
disturbing the soil or changing the water-content; (2) activities which pro- 
duce a dry or drier habitat, usually with much disturbance of the soil; (3) 
activities which produce a wet or wetter soil or a water area. There is clearly 
no sharp line of demarcation between the three groups, but this is evidence 
that the distinction is a natural and not an artificial one. The simplest and 
most convenient arrangement is one based upon agents and kinds of activity 
(Clements 1904:116; 1905:249; 1907:279), but this is not in fundamental 
relation to successional development. 

Bare areas due to destruction of vegetation alone. — The primary activities 
by which man produces denuded areas are burns and clearings. Clearings 
result for the most part from lumbering or from cultivation, though a host of 
minor activities have the same result. Ant areas in arid regions are perhaps 
the best examples of clearing by animals without soil disturbance. In all cases 
of burning and clearing the intensity or thoroughness of the process deter- 
mines whether the result will be a change of vegetation or the initiation of a 
sere. The latter occurs only when the destruction of the vegetation is com- 
plete, or so nearly complete that the pioneers dominate the area. Lumbering 
consequently does not initiate succession except when it is followed by fire or 
other process which removes the undergrowth. Most fires in woodland 
denude the burned area completely, but surface fires and top fires merely 
destroy a part of the population. Fires in grassland practically never pro- 
duce bare areas for colonization. Poisonous gases from smelters, factories, 
etc., sometimes result in complete denudation, though the action is chiefly felt 
in a change of vegetation. Cultivation normally results in complete destruc- 
tion of the original vegetation. In the broadest sense, a new sere starts with 
the sowing or planting of the crop. In the case of annual crops, however, 
real development begins only when cultivation is abandoned. In new or 
sparsely settled grassland regions, the wearing of roads or trails results in a 
characteristic denudation with little or no soil disturbance. Complete denuda- 
tion by animals is only of the rarest occurrence, except where they are 
restricted to limited areas by man. Even in striking cases of the destruction 
of a forest by parasites, such as the repeated defoliation of aspens by cater- 
pillars, the undergrowth is little affected. Complete destruction by parasites 
usually occurs only in the case of annual crops. A striking example of 
denudation by a plant parasite was found on the shores of False Bay in 
southern California, and especially on the dunes of Medanos Spit. Here 
families and colonies of Abronia umbellata and Franseria bipinnatifida were 
completely covered with an orange mat of Cuscuta salina. The dodder in 
May had already killed many of the families entirely, and it was obvious 
that many more would suffer the same fate. With the gradual death of the 
hosts, the dodder became brown and dried up with the host plants. The two 
were then gradually blown away by the constant onshore winds and a bare 
sand area was left. 

Bare areas with dry or drier soils. — These occur chiefly where there is a 
marked disturbance of the soil. The latter affects the water-content by chang- 
ing the texture, by changing the kind of soil, as from clay to sand or gravel, 
or by both methods. These results may be produced by removal, by deposit, 
or by the stirring of the soil in place. In the case of man they are produced 



60 INITIAL CAUSES 

by the widest variety of construction and engineering processes, with roads 
and railroads as universal examples. The removal and deposit of soil by 
animals is confined to the immediate neighborhood of the burrows of rodents, 
the homes of ants, etc. In some cases, such as densely populated prairie-dog 
towns, the burrows are sufficiently close to produce an almost completely 
denuded area. Insignificant as most areas of this sort are, they give rise to 
real though minute seres of much value in communities otherwise little 
disturbed. 

Bare areas with wet soils or water. — As indicated under topographic 
causes, draining and flooding may bring two different areas to the same con- 
dition for invasion. The habitats produced by both are similar in having a 
wet soil, capable of colonization only by hydrophytes or marsh-plants, except 
in cases where drainage is reinforced by rapid or excessive evaporation. This 
is true of the canals and ditches, as well as of the areas actually drained or 
flooded, and equally so of all canals and ditches, regardless of their purpose. 
Again, it is unimportant whether flooding, for example, is brought about by 
the diversion of a stream of water or by the construction of a dam. It is 
equally immaterial whether the dam is built by man or by beavers. The 
essential fact is that the water-content will be excessive and that the pioneer 
stages will consist of hydrophytes in all these cases. The effect of drainage, 
i. e., relative lowering of the water-level, can be produced by filling, just as 
flooding can be caused by the formation of a depression due to the removal 
of soil. An exceptional instance of the former is furnished by the case of 
coral reefs and islands. 

PRIMARY AND SECONDARY AREAS. 

Distinction. — The whole course of succession rests upon the nature of the 
bare area which initiates it. We have already seen that the essential nature 
of a bare area is expressed in the amount and kind of water. Hence, in 
attempting to group naturally all the foregoing areas, i. e., from the stand- 
point of succession, it is necessary to recognize that water areas and rock areas 
constitute the two primary groups. While these are opposed in water-content 
and density, they agree in presenting extreme conditions in which develop- 
ment is necessarily slow and of long duration. The denudation of either area 
in the course of succession results in the sudden reappearance of earlier con- 
ditions, which cause the repetition of certain stages. If denudation consists 
in the destruction of the vegetation alone, the soil factors are changed rela- 
tively little. The sere thus initiated is relatively short, consisting ef fewer 
stages and reaching the climax in a short time. If the soil is much disturbed, 
however, the conditions produced approach much nearer the original extreme, 
and the resulting sere is correspondingly longer and more complex. The 
degree of disturbance may be so great as to bring back the original extreme 
conditions, in which case the normal course of development is repeated. This 
amounts to the production of a new area, both with respect to the extreme 
condition and the lack of germules. Hence, all bare areas fall into a second 
basic grouping into primary and secondary areas. Primary bare areas pre- 
sent extreme conditions as to water-content, possess no viable germules of 
other than pioneer species, require long-continued reaction before they are 
ready for climax stages, and hence give rise to long and complex seres. Sec- 



CLEMENTS 








[ ' 













A. Primary area colonized by mosses, terminal moraine of the Illecillewaet Glacii 

Glacier, British Columbia. 
B. Secondary area eolonized by Sdtsola, on a railway embankment, bad lands, 
Scott's Bluff, Nebraska. ' 



PRIMARY AND SECONDARY AREAS 61 

ondary bare areas present less extreme conditions, normally possess viable 
germules of more than one stage, often in large number, retain more or less 
of the preceding reactions, and consequently give rise to relatively short and 
simple seres. From the standpoint of succession, secondary areas are related 
to primary ones. In consequence, the most natural classification of all bare 
areas seems to be into primary and secondary, with a subdivision into water, 
rock, and soil (plate 6, a, b). 

Sterility of primary and secondary areas.— As stated above, primary areas, 
such as lakes, rocks, lava-flows, dunes, etc., contain no germules at the outset, 
or no viable ones other than those of pioneers. Secondary areas, on the con- 
trary, such as burns, fallow fields, drained areas, etc., contain a large number 
of germules, often representing several successive stages. In some cases it 
seems that the seeds and fruits for the dominants of all stages, including the 
climax, are present at the time of initiation. The sterility of the soil of a pri- 
mary area is due chiefly to the relatively long period of its formation, and to 
the effect of excessive water-content or drouth upon migrating germules. In 
all cases it arises in a measure also from the impossible conditions for the 
ecesis of all plants except pioneers. In these points most secondary soils offer 
a sharp contrast. The method of origin permits the persistence of seeds or 
perennial parts or both, and its suddenness usually allows the immediate 
entrance of many migrants. The soil affords favorable conditions for the 
preservation of seeds and fruits, often for many years, as of course for ready 
ecesis (plate 9, a, b). 

This contrast between primary and secondary areas is seen most strikingly 
in the case of land-slips, where the slip exposes rock on the mountain side and 
produces a mass of soil and vegetation at the bottom. This is sometimes true 
also of the fragmentation of cliffs by gravity and of erosion and deposit due to 
torrential rains or other agents which act suddenly. 

Denudation. — Secondary areas are the result of denudation, with or with- 
out the disturbance of the soil. Their nature is dependent upon the process 
of denudation and upon the degree to which it acts. The latter is ordinarily 
much the more important. It determines in the first pdace whether the result 
will be merely a change in the existing community or the production of a bare 
area. In the case of the complete removal of vegetation, as by fire, the soil 
may be disturbed so little that it offers essentially the same conditions as 
before denudation, and initiates a sere correspondingly brief and simple. On 
the other hand, the disturbance of the soil may operate to various depths and 
produce correspondingly extreme conditions up to the final extremes, water 
and rock, which constitute new areas. The production of new areas by 
denudation and soil disturbance is relatively infrequent, however. 

Methods of denudation. — Denuding forces operate normally by the 
destruction of vegetation, accompanied by the disturbance or removal of the 
soil. Destruction may, however, be a consequence of flooding or deposit. 
Apart from the destruction of the existing population, it is the depth of 
removal or deposit of soil which is critical. The rate of removal or deposit 
often plays an important part also, though it is usually expressed in depth. In 
burns there is practically no disturbance of the soil at all, though its composi- 
tion may be materially affected. Cultivation disturbs soil, changing its tex- 
ture and water-content in different degrees. Construction and engineering 



62 INITIAL CAUSES 

operations effect removal and deposition of the soil in varying degree. Because 
of its action in destroying vegetation, water must be considered in this connec- 
tion also, especially in the case of flooding. Climatic initial causes produce 
denudation alone, while topographic ones exhibit the same wide range of effect 
shown by biotie causes. 

Depth of removal or deposit. — The reaction of plants upon the soil is con- 
fined wholly or chiefly to the layer in which the roots grow. This depth estab- 
lishes the limit to which removal may ordinarily go without changing soil 
conditions essentially. In the early stages of very loose soils, such as the sand 
of bars and dunes, the reaction is slight, but it seems probable, however, that 
these, too, must follow the general rule, namely, that the removal of the soil 
built up by reaction must necessitate a return to primary conditions. In the 
vast majority of cases a secondary area is formed whenever removal operates 
within the root layer of soil. This may be readily tested by instrumental 
methods or by experiment. In general, the composition of the initial stage 
of the sere indicates this clearly enough. The removal of this layer to differ- 
ent depths is reflected in the composition and length of the resulting secondary 
sere or subsere. 

In cases where the destruction of the vegetation is accompanied or followed 
by the deposition of soil, the nature of the bare area will be decided by the 
kind of soil deposited as well as by its depth. If sand or gravel are laid down 
over loam to a sufficient depth, the water relations of the area may be moved 
to one extreme and a primary habitat result. Here the depth must approxi- 
mate the length of the root system of the species of the initial stages. Other- 
wise the roots will reach the original soil and the development will be con- 
trolled in some degree by the latter. When the depth of added soil exceeds 1 
or 2 meters, a secondary succession can result only when the soil is essentially 
similar in texture and water relations to the original. This is apparently 
true in the majority of cases (plate 7, A, b). 

The effects of the removal of water by drainage or of the addition of water by 
flooding may be alike or unlike. Either flooding or drainage may destroy a plant 
population and yet leave the area little changed, thus initiating a secondary 
succession. This is the regular effect of drainage when it does not merely 
modify the existing vegetation. In the case of ponding, however, the water 
produces a new set of extreme conditions, and this constitutes a primary area. 

Rate and extent of removal. — Destruction of a community with accom- 
panying or subsequent removal of the soil is the general process of which topo- 
graphic erosion is much the most important part. In fact, erosion may well be 
regarded as the general process, which is produced by topographic, climatic, 
or biotie forces. While depth is the final criterion of the effect of erosion, both 
its rate and extent have an influence. Erosion to a depth of a foot would pro- 
duce different conditions when caused by a single torrential rain from those 
due to gradual erosion spread over several years, though in both eases the 
resulting area would normally be a secondary one. The differences would con- 
sist as much in the stability of the surface for migration and ecesis as in the 
water relation. The extent of the denuded area is closely related to depth of 
erosion. When the latter is local, it is less apt to depart widely from the nor- 
mal condition, and its invasion is controlled almost completely by the parent 
area. This matter is discussed in detail in the section on cycles of erosion. 



CLEMENTS 




«£->■:£• ..-,*>**.- <?ife^"-i*» »>>^^ w v saSSr 2 ** 1 ^ 



jiff 









,-^fc 








A. Superficial wind erosion, Dune Point, La Jolla, California. 
B. Deep-seated water erosion, Torrey Pines, Del Mar, California. 



IV. ECESIC CAUSES. 

Nature. — As has been indicated, succession owes its distinctive character 
to the communities which succeed each other in the same area. This character 
is given it by the responses or adjustments which the community makes to 
its habitat, namely, migration, ecesis, competition, and reaction. These are 
the real causes of development, for which a bare area does little more than fur- 
nish a field of action. To them is due the rhythm of succession as expressed 
in the rise and fall of successive populations. They may well be regarded as 
the paramount causes of succession, since their action and interaction are the 
development of vegetation. As every sere must begin with a denuded area 
and end in a climax, it is clearer to treat them along with initial causes and 
climax causes. 

AGGREGATION. 

Concept and role. — Aggregation is the process by which germules come to 
be grouped together (Clements, 1905:203; 1907:237). It consists really of 
two processes, simple aggregation and migration. These may act alone or 
together, but the analysis is clearer if each is considered separately. By sim- 
ple aggregation is understood the grouping of germules about the parent 
plant. Even in the fall of seeds there is often some movement away from the 
parent plant, but it can not properly be regarded as migration, unless the seed 
is carried into a different family or into a different portion of the same colony 
or clan. The distinction is by no means a sharp one, but it rests upon two 
factors of much importance in vegetation. The first is that movement within 
the parent area bears a different relation to ecesis from movement beyond the 
parent area. The second fact is that simple aggregation increases the indi- 
viduals of a species and tends to produce dominance, while migration has the 
opposite effect (plate 8 a). 

Simple aggregation may operate by seeds and fruits, by propagules, or by 
both. The method of aggregation plays an important part in determining 
the germules in secondary areas, and in the initial stages of a sere. In this 
respect it is essentially like migration, and will be considered in connection 
with the discussion of the parts used as migrules. 

Effects of simple aggregation. — Aggregation usually modifies the compo- 
sition and structure of existing communities. This effect is seen most strikingly 
where the vegetation is open, though it is readily disclosed by the quadrat in 
closed communities. The increase of population in the case of the pioneers 
of a bare area is mainly a matter of aggregation. Conspicuous examples of 
this are found in areas with unstable soil, such as gravel-slides, blow-outs, 
bad lands, etc. The influence of aggregation is especially important in com- 
munities which are destroyed by fire, cultivation, etc. In many instances 
the change in soil conditions is slight, and the course of succession is deter- 
mined by the number of germules which survive. If the number is large, as 
in certain forest areas, the resulting sere is very short, consisting only of the 
stages that can develop while the trees are growing to the size which makes 
them dominant. When the number of aggregated germules is small or none, 
the selective action of migration comes into play, and the course of develop- 
ment is correspondingly long. 

63 



64 ECESIC CAUSES 

Relation to denuded areas. — Aggregation is the normal result of seed-pro- 
duction in a community. Its importance in secondary areas depends wholly 
upon whether it occurs before or after the action of the denuding agent. 
Normally, of course, it occurs before denudation, and the question is chiefly 
one of the kind and number of germules which escape destruction. This is 
determined by the agent, the position of the germule, and sometimes by its 
nature. In the case of fire, seeds and fruits on the surface or near it are 
destroyed, unless they have unusual protection, as in some woody cones. 
Fruits buried by rodents, or seeds and fruits which become covered with moist 
duff, often survive. In cultivated areas, seeds often persist for a long time, 
though they play no part in succession unless they survive until the field is 
abandoned. On the other hand, intensive cultivation destroys all under- 
ground parts, while fire has little or no effect upon them. In grassland the 
effect is merely to modify the population, but in woodland succession results. 

Aggregation occurs after fire only in a few striking instances. It occurs in 
the case of many conifers with large or hard cones, especially where the fire 
kills the trees but leaves them standing. This is often true of lodgepole pine 
(Pinus murrayana), jack pine (P. divaricata) , and all others in which the 
cones remain closed and attached to the branches for a long time. 

Interaction of aggregation and migration. — All sterile bare areas owe their 
pioneers to migration. After the establishment of the first invaders the devel- 
opment of families and colonies is due primarily to aggregation (plate 8 b). 
The appearance of each successive stage is caused by the interaction of the 
two processes. Migration brings in the species of the next stage, and aggrega- 
tion causes them to become characteristic or dominant. Their relation in each 
stage is shown in the development of the succession as a whole. Migration 
marks the beginning of the sere, as of each stage. It becomes relatively more 
marked for a number of stages, and then falls off to a minimum. In dense 
closed forests it becomes extremely rare, and the ecesis of the migrants impos- 
sible. On the other hand, aggregation becomes more marked with successive 
stages, and a sere may end in what is essentially a family, e. g., a pure stand 
of Psevdotsuga or Picea with practically no undergrowth. 

MIGRATION. 

Concept. — The nature of migration as an essential process in succession 
has been analyzed in detail elsewhere (Clements, 1904:32; 1905:210; 1907: 
240) . It will suffice to summarize the main points in connection with indicat- 
ing their special bearing upon the nature and course of succession. The use of 
the term is restricted to its proper sense of movement. Migration is regarded 
as a process distinct from establishment or ecesis. The two are most intimately 
related in the general process of invasion, which comprises movement into a 
habitat and establishment there. Migration begins when a germule leaves the 
parent area and ends when it reaches its final resting-place. It may consist 
of a single movement, or the number of movements between the two places 
may be many, as in the repeated flights of pappose and winged fruits. The 
entrance of a species into a new area or region will often result from repeated 
invasions, each consisting of a single period of migration and ecesis. 

Mobility. — Mobility is the ability of a species to move out of the parent 
area. Among terrestrial plants, it is indicated chiefly by the size, weight, and 



MIGRATION 65 

surface of the disseminule. This is particularly true of seeds and fruits 
carried by wind and water. Man and animals distribute fruits for so many 
reasons and in so many ways that the only test of mobility in many cases is 
the actual movement. This is especially clear in the case of many weeds of 
cultivated fields, which owe their migration wholly to their association. Mobil- 
ity is also directly affected by the amount of seed produced. It is increased 
by large seed-production, both on account of the large number of seeds or 
fruits and the correspondingly smaller size. 

The relation of mobility to succession is obvious. In bare land areas, and 
especially in denuded ones, the order of appearance of species is largely a 
matter of the size and modification of the disseminule. The earliest pioneers — 
lichens, liverworts, and mosses — usually have microscopic germules, whether 
spores, soredia, or gemmae. The early herbaceous pioneers are grasses and 
herbs with small seeds and fruits, well adapted for wind-carriage, as in fire- 
grass (Agrostis hiemalis) and fire-weed (Chamaenerium angustifolium) , or 
mobile by virtue of association, as in Brassica, Lepidium, Chenopodium, etc. 
The sequence of shrubby species is determined partly by mobility, as is true 
of Rubus in burns, Salix in lowlands, and Cercocarpus in grassland. The 
same relation is shown in trees by the fact that Populus and Betxda are every- 
where woodland pioneers. Trees constitute the climax life-form, however, and 
their successional relation is chiefly due to other factors. 

Seed-production. — The absolute seed-production of a species bears a gen- 
eral relation to its power of invasion. The latter is expressed more exactly by 
the efficient seed-production, which is the total number of fertile seeds left 
after the usual action of destructive agents. The number of seeds produced 
by a tree of Firms flexilis is large, but the efficiency is almost nil. The toll 
taken by nut-crackers, jays, and squirrels is so complete that no viable seed 
has yet been found in hundreds of mature cones examined. The fertility of 
seeds is greatest in typical polyanthous species which produce but one seed per 
flower, such as grasses, composites, and other achene-bearing families. This 
is shown by the large number of successful invaders, i. e., weeds, produced by 
these groups. Fertility is often low in polyspermous plants, due to the lack 
of fertilization or to competition between the ovules. The number of seeds is 
often correlated with size, but the exceptions are too numerous to permit the 
recognition of a general rule. The periodic variation in the total seed- 
production is a factor of much importance, especially in trees and shrubs. 
This is due to the fact that birds and rodents consume practically the entire 
crop in the case of conifers, oaks, etc., during poor seed-years. The efficient 
production is high only during good years, and the invasion of such species is 
largely dependent upon the occurrence of good seed-years. 

The influence of seed-production is felt in mobility, in ecesis, and in domi- 
nance. Its effect can only be estimated at the present, owing to the lack of 
exact study. It is probable that the quantitative investigation of the seed- 
production of dominant and characteristic species will go far towards reveal- 
ing the real nature of dominance. 

Influence of the organ used. — When runners, stolons, and rhizomes carry 
buds several to many feet from the parent plant, the result may well be 
regarded as migration rather than aggregation. Such migration plays a small 
part in the colonization of new areas. It is almost negligible in comparison 



66 ECESIC CAUSES [ 

with the migration of free parts, such as spores, seeds, and fruits, especially in 
large areas. Naturally, species which are readily carried by seeds and fruits, 
and move also by offshoots, form excellent pioneers. The influence of size of 
organ is indicated by the relative mobility of spores, seeds, and fruits. In spite 
of many exceptions, spores are more readily and widely distributed than seeds, 
and seeds than fruits. This is shown in some measure also by the success in 
migration of plants in which the fruit simulates a seed almost perfectly, as in 
the grain, achene, etc. The handicap of the fruit in regard to size is often 
counterbalanced by the perfection of the contrivance for dissemination. In 
the case of tumbleweeds and tumbling grasses, the whole plant or the major 
portion of it has assumed a form which amounts practically to a nearly per- 
fect contrivance for effective migration (plate 8 c). 

Influence of the migration contrivance. — The effect of the modification for 
carriage is intimately blended with that of the agent, as would be expected. 
The perfection of the device determines the success of the agent, as is well 
seen in those modifications which increase the surface for wind carriage. 
Sack and bladder fruits, as in Physalis, are relatively ineffective, and are 
often associated with other devices. Wings give greater buoyancy, but are 
only moderately efficient, except when the seed or fruit is small and light. 
The vast majority of samaras of the elm, maple, ash, etc., fall near the parent 
tree. This is strikingly true of the seeds of conifers. A careful transect study 
of the flight of seeds of the spruce and the fir showed that practically all of 
them landed within a distance equal to the height of the tree. Comate and 
pappose seeds and fruits are by far the most efficient of wind-borne dissemi- 
nules, and probably of all kinds as well. Here again success is determined 
largely by smallness of size, but apart from this the perfection of the device as 
to the number, length, and position of scales or hairs is decisive. Scales are less 
efficient than bristles or hairs, and the latter are successful in proportion to 
length and number. Disseminules tufted at one end are carried more readily 
than those covered with hairs, and a pappus which spreads widely or is plumy 
is the most effective of all. The relative efficiency of devices for carriage by 
animals is less evident, but the number of pioneers which possess fruits with 
spines or hooks is significant. 

Many fruits migrate readily, even when the migration device is not greatly 
perfected. This is due to the fact that they avail themselves of two or more 
agents, either by means of two distinct devices or because of their behavior on 
drying. In Physalis the bladdery fruit is rolled over the ground by the wind, 
and then the seeds scattered by birds and rodents. Stipa, Erodium, and other 
plants with sharp-pointed twisting fruits, are carried by attachment and 
blown by the wind in tangled clusters, the two agents often alternating many 
times. A striking case of this sort is afforded by Micrampelis, which is a 
frequent pioneer in denuded areas along streams. The fruits are blown by the 
wind, floated by streams, and even carried by attachment, while the seeds, 
in addition to being forcibly expelled, are readily carried by water. 

The distance of migration is a direct consequence of the perfection of the 
device. Hence the latter is of the first importance in selecting the migrants 
which are moving toward a new area. It thus plays a large part in determin- 
ing what species will enter it as pioneers, as well as the stages in which others 
will appear. The comate seeds of fireweed, aspen, and willow may be carried for 



CLEMENTS 





i^ 









■ M 






*sms8* 



ft : 







SwSkJL'. _...-..-' 



A. Family of Pachylophus caespitosus on gravel-slide, Alpine Laboratory, Colorado. 

I>. Colony of Suaeda and Atriplu- in n depression, bed of a former salt lake, Ha/on. Nev. 

C. Tumbleweed, Saisola, on the Great Plains, Akron, Colorado. 



MIGRATION 67 

at least several miles in such quantities as to produce dominance. Dominance 
in the development in secondary areas, especially, is directly dependent upon 
the number of seeds which enter, and hence upon the migration device. If 
seeds or one-seeded fruits migrate singly, the resulting individuals stand 
separated, and dominance results only from the movement of large numbers. 
In a relatively large number of cases, several-seeded or even many-seeded fruits 
migrate, and upon germination produce the nucleus of a community. Often, 
also, fruits become tangled with each other, as in Stipa, Erodium, Xanthium, 
Desmodium, etc., and are transported to new areas, when they produce families. 
This is particularly true of tumble-weeds (Salsola, Cycloloma, Amaranthus, 
etc.), and of tumble-grasses (Panicum capillar -e, Eragrostis pectinacea, etc.). 

Role of migration agents. — It is significant that the agents which carry 
migrules, viz, wind, water, gravity, glaciers, man, and animals, are also the 
initial causes of bare areas. Thus, the force which produces an area for suc- 
cession also brings the new population to it. Often the two processes are 
simultaneous, especially in denuded habitats. The relation is as simple as it 
is intimate. Water as a migration agent brings to new water or soil areas 
chiefly those germules which can be gathered along its course. Thus it is self- 
evident that a new area with an excess of water will be provided for the most 
part with water-borne migrules, and that the viable ones will practically all 
be of this kind. The action of wind is broader, but it is clear that initial areas 
due to wind are found only in wind-swept places, which are of course where the 
wind will carry the largest load of migrules. An extremely close connection is 
found also in the talus slopes due to gravity, for the majority of the species are 
derived from above. The universal prevalence of ruderal plants in denuded 
areas due to man's activities is sufficient evidence of the direct relation here. 

Destructive action of agents. — The action of water upon seeds practically 
eliminates all but hydrophytic or ruderal species as pioneei's in water or wet 
areas (Shull, 1914:333), though this effect is doubly insured by the difficulties 
of ecesis. Large quantities of seeds are also destroyed in all areas produced by 
deposit, and especially in talus. The action of seed-eating agents, particularly 
birds and rodents, is often completely decisive. This is seen most strikingly in 
secondary areas, but it occurs in all places where seeds are exposed. So com- 
plete is the destruction of seeds in certain instances, notably in forests of 
lodgepole pine, that the reappearance of certain species is possible only where 
the rodent population is driven out or destroyed. This is confirmed by the 
almost uniform failure of broadcast sowing in reforestation, as well as in other 
methods of sowing when the birds and rodents are not destroyed. No other 
factor in invasion has been so often overlooked, and its exact value is conse- 
quently hard to determine. If the few quantitative results so far obtained are 
representative, it must be regarded as of great and often of critical impor- 
tance. 

Direction of migration. — While migration tends to radiate in all direc- 
tions from the parent group, it often comes to be more or less determinate. In 
general, it is radial or indeterminate when it is local, and unilateral or deter- 
minate when more general. The local movement due to wind, man, or animals 
may be in any and all directions, while distant migration by either agent will, 
usually be in one direction. This is peculiarly true of carriage by streams, 
in which the regular movement is always down the valley. In the floristic 



68 ECESIC CAUSES 

study of vegetation, distant migration has appeared more striking and inter- 
esting than local. It is in no degree as important in the study of succession, 
as local migration is primarily responsible for the population of new areas. 
Here, again, exact observations and experiments are few, but most of the evi- 
dence available shows that effective invasion in quantity is always local. This 
is doubtless true of great migrations such as those of the glacial and post- 
glacial times, when population moved hundreds of miles. These were appar- 
ently only the gross result of repeated local movements, acting in the same 
general direction through long periods. 

Up to the present time the study of succession has been almost wholly con- 
fined to examining and correlating communities during one or a few seasons. 
The development has not been followed in the various portions of its course, 
but has been reconstructed from the end results, i. e., the communities. While 
the whole course of a primary sere can be obtained in no other way, every one 
of its stages permits quantitative study of its own development. Secondary 
seres may often be studied as processes in their entirety, owing to their much 
shorter course. In such work the position of the bare area with reference to 
the migration agents active is of the first consequence. An area surrounded 
by a community of the successional series will be quickly colonized by immi- 
gration from all sides. One lying in the ecotone between two associations will 
have its development influenced by the prevailing direction of movement. 
This is well illustrated by the behavior of new areas just below timber-line on 
mountains. The area belongs to the forest climax, but it is invaded and held 
by alpine species for a very long time, if not permanently. This is due to 
the ease with which seeds and fruits from the alpine area above are brought to 
the area by gravity, and to the extreme difficulty the forest migrules find in 
moving up the slope. Man and animals are the only agents which can over- 
come this effect. The only exception is furnished by small comose seeds, such 
as those of the fireweed and aspen, which may be carried hundreds of feet up 
mountain sides by the wind. 

ECESIS. 

Nature and role. — Ecesis is the adjustment of the plant to a new home 
(Clements, 1904:50; 1905:220; 1907:261). It consists of three essential 
processes, germination, growth, and reproduction. It is the normal conse- 
quence of migration, and it results sooner or later in competition. Ecesis 
comprises all the processes exhibited by an invading germule from the time it 
enters a new area until it is thoroughly established there. Hence it really 
includes competition, except in the case of pioneers in bare areas. The ecesis 
of a social plant is the same as that of an isolated invader in essentials, but it 
takes place under conditions modified by the neighboring plants. Hence it 
promises clearer analysis if ecesis is considered first and competition subse- 
quently. 

Ecesis is the decisive factor in invasion. Migration is wholly ineffective 
without it, and at present, indeed, is usually measured by it. The relation 
between the two is most intimate. Ecesis in bare areas especially depends in 
a large measure upon the time, direction, rapidity, distance, and amount of 
migration. There is usually an essential alternation between the two, since 
migration is followed by ecesis, and the latter then establishes a new group 



ECESIS 69 

from which further migration is possible, and so on. The time of year in 
which fruits ripen and migration occurs has a marked influence upon the 
establishment of a species. Migrules ordinarily pass through a resting-period, 
but are frequently brought into conditions where they germinate at once and 
then perish, because of unfavorable conditions, or because of competition. 
The direction and distance of movement are decisive in so far as they deter- 
mine the kind of habitat into which the seed is carried. The number of mi- 
grants is likewise important, since it affects the chances that seeds will be 
carried into bare areas where ecesis is possible. 

In the case of algas, migration and ecesis become nearly or quite synony- 
mous, since plants of this sort are at home almost anywhere in the water. 
Indeed, it may be said that they are always at home, because they remain 
in the same habitat, no matter where carried. With aquatic flowering plants 
the case is somewhat different. The plants when free behave much as alga? 
do in regard to ecesis, but each new individual has to go through the processes 
of germination and growth. This is similar to what occurs in the aggregation 
of land plants. The seeds or underground buds do not find themselves in a 
new home exactly, but, apart from the greater certainty of success, the course 
of ecesis is the same. 

The term ecesis, from the Greek oik^o-is, the act of coming to be at home ; 
hence, adjustment to the habitat, or oIkos, was first proposed (Clements, 
1904:32) to designate the whole process covered more or less completely by 
acclimatization, naturalization, accommodation, etc. It has proved so definite 
and convenient in use that it seems desirable to employ a corresponding verb, 
ecize from oIki£<d, to make a home, colonize. 

(Termination. — The first critical process in ecesis is germination. The 
ejxact scope of germination is debatable, but in nature it is most convenient to 
regard it as including the appearance and unfolding of the first leaf or leaves, 
whether cotyledons or not. It occurs regularly when a viable seed meets favor- 
able conditions as to water, heat, and oxygen. It is often delayed or even 
absent when the seeds of native species are first sown under cultivation, and it 
is probable that germination is often delayed in nature, even when conditions 
seem favorable. A viable seed must contain a normal embryo, capable of 
absorbing water, and using the stored food for growth and consequent escape 
from the seed-coats. The amount of water, heat, and oxygen present must 
suffice to bring the seedling to the point where it can make food and begin its 
own independent existence. Hysterophytes are naturally exceptions. 

With the exception of seeds of forest trees and certain ruderals, we have 
practically no accurate knowledge of the gei*minability of native species, 
especially at those times when conditions favor germination. The normal 
period of viability \mder the usual conditions of natural sowing is also un- 
known, as well as viability under extremely favorable and unfavorable condi- 
tions. In most cases the period of duration is a function of the seed-coats or 
pericarp, but in some viability is inherent in the embryo itself. The control of 
the habitat is two-fold. It determines whether the seed will germinate either 
immediately or during the season. If germination is delayed, it determines 
whether conditions will permit the seed to remain dormant but viable for 
several years. Habitats which are most favorable to germination are least 
favorable to dormant seeds, and, conversely, those which allow seeds to persist 



70 ECESIC CAUSES 

for long periods are inimical to germination. In many cases, of course, the 
surface layer favors germination, and deeper layers, persistence. 

Successful germination usually occurs only at proper depths, with the excep- 
tion of bare areas with wet or moist surfaces. A few species have the peculiar 
property of being able to plant themselves when they germinate on the sur- 
face, but the rule is that seeds must be covered with soil to permit ecesis. 
This is particularly true of seeds on a forest-floor covered with a thick layer 
of leaves or needles, which prevent the root from striking into the soil. There 
is doubtless an optimum depth for each species, which varies more or less 
with the habitat. Too great a depth prevents the seedling from appearing 
altogether, or causes it to appear in such abnormal condition that it quickly 
succumbs. In the former case it may lead to dormancy, and germination 
after the area has been cleared or burned. The effect of depth and its relation 
to size of seed has been shown by Hofmann (1916) in the case of conifers. In 
Pinus pmderosa, with the largest seeds, 96 per cent germinated and 86 per 
cent appeared above ground at a depth of 1 inch, while only 36 per cent ger- 
minated and none appeared at 4 inches deep. In the case of Pseudotsuga, 93 
per cent germinated and appeared at 0.5 inch, but only 17 per cent germinated 
at 4 inches and none appeared. For Tsuga heterophylla, at 0.25 inch the per- 
centage was 96 and at 1.25 inches 42 per cent and 0, and for Thuja plicata, 
with the smallest seeds, 78 per cent at 0.12 inch and 26 per cent and at 1 
inch. The same investigator found that seeds of Pinus monticola, Pseu- 
dotsuga, and Tsuga heterophylla remained dormant in the soil for 6 years, 
those of Taxus brevifolia for 8, Abies amabilis for 5, A. nobilis for 3, and 
Thuja plicata for 2 years. While this is a relatively short time in comparison 
with the period in some ruderal species, it is of much more significance in 
succession. 

Fate of seedling. — The crucial point in ecesis is reached when the seedling 
is completely freed from the seed-coat and is thrown upon its own resources 
for food and protection. Even before this time, invading seedlings are often 
destroyed in great numbers by birds and rodents, which pull them up for the 
food supply still left in the seed-coats. The tender seedlings are often eaten 
by the smaller chipmunks, and sometimes coniferous seedlings seem to be 
pulled up or bitten off in mere wantonness. In regions where grazing occurs, 
the destructive action of the animals is very great, especially in the case of 
sheep. Some toll is taken by damping-off fungi, such as Pythium and 
Fusarium, in moist, shady soils, but these are perhaps never decisive, except 
in artificial conditions. In the case of herbs, the greatest danger arises from 
excessive competition, especially in the dense aggregation typical of annuals. 
The direct effect is probably due to lack of water, though solutes and light 
may often play a part. With the seedlings of woody plants the cause of the 
greatest destruction is drouth in midsummer or later. This is the primary 
factor in limiting the ecesis of many conifers, though the "heaving" action 
of frost is frequently great or even predominant. The root-system is often 
inadequate to supply the water necessary to offset the high transpiration 
caused by conditions at the surface of the soil. Moreover, it is likewise too 
short to escape the progressive drying-out of the soil itself. In open places 
in the Rocky Mountains, such as parks, clearings, etc., the late summer mor- 
tality is excessive, often including all seedlings of the year. On the forest- 



ECESIS 71 

floor itself it is considerable or even decisive in places where a thick layer of 
dry mold or dust increases the distance roots have to go. Shreve (1909:289) 
has found that the seedling mortality of Parkinsonia in the deserts of Arizona 
was 70 per cent during the first year and 97 per cent by the end of the third 
year. 

Growth. — If the seedling establishes itself, it is fairly sure to develop. This 
seems to be the rule with herbaceous plants, though it suffers some exceptions 
in the case of trees and shrubs. Even though conditions become more extreme, 
the old plant is usually better able to resist them. With increasing size of 
individuals the demands increase correspondingly. Hence, growth causes an 
increasing competition. Out of this competition some species emerge as 
dominants, reacting upon the habitat in a controlling way and determining 
the conditions for all other species in the community. Others represent an 
adaptation to conditions caused by the dominance and play always a subor- 
dinate part, A third behavior is shown by those species or individuals ordi- 
narily capable of becoming dominant, whenever they appear tardily, or repro- 
duce under unfavorable light intensities. The growth is diminished and the 
plant becomes suppressed. In forest and thicket suppression is progressive, 
and usually results in death, either through insufficient nutrition or in conse- 
quence of the attacks of insects and fungi. While suppression occurs in all 
degrees, its most important effect lies in inhibiting reproduction, and it would 
be well if the term were restricted to this sense. 

Reproduction. — The invasion of a bare area is made possible by reproduc- 
tion or seed-production in the neighboring communities. The development of 
each stage in the resulting sere is the consequence of the excess of reproduc- 
tion over immigration. Reproduction is in consequence the final measure of 
the success of ecesis. In terms of succession at least, ecesis occurs only when 
a species reproduces itself, and thus maintains its position throughout the 
stage to which it belongs. In changes of vegetation the total period of ecesis 
may be much shorter ; in fact, annuals may appear and disappear finally in 
a single season. In the case of annuals it is evident that there is no ecesis 
without reproduction. With perennials it is less clear, but there are few 
species that can maintain themselves in an area by vegetative propagation 
alone. Since bare areas are rarely invaded in this way, complete ecesis in 
them must rest upon reproduction. 

Ecesis in bare areas. — The selective action of bare areas upon the gernmles 
brought into them is exerted by ecesis. It has repeatedly been pointed out 
that the essential nature of such areas is found in the water relations, and 
that it can best be expressed in the amount of departure from the climatic 
mean. The two extremes, water and rock, are the extremes for ecesis. the one 
impossible for plants whose leaves live in the air and the light, the other for 
those whose roots must reach water. The plants which can ecize in such 
extremes are necessarily restricted in number and specialized in character, 
but they are of the widest distribution, since the habitats which produced 
them are universal. From the standpoint of ecesis, succession is a process 
which brings the habitat nearer the optimum for germination and growth. 
and thus permits the invasion of an increasingly larger population. The 
fundamental reason why primary succession is long in comparison with 
secondary lies in the fact that the physical conditions are for a long time 



72 ECESIC CAUSES 

too severe for the vast majority of migrants, as well as too severe for the 
rapid increase of the pioneers. Secondary soils, on the contrary, afford more 
or less optimum conditions for germination and growth, and are invaded and 
stabilized with corresponding rapidity (plate 9, a. b). 

COMPETITION. 

Nature. — Competition occurs whenever two or more plants make demands 
in excess of the supply (Clements, 1904:166; 1905:285; 1907:251). It is a 
universal characteristic of all plant communities, and is absent only in the 
initial stages of succession, when the pioneers are still isolated. It increases 
with the increase of population in successive stages until the climax or sub- 
climax is reached, after which it decreases again with the population. It is 
necessarily greatest between individuals or species which make similar or 
identical demands upon the same supply at the same time, and least or quite 
lacking in associated plants with demands largely or quite unlike. 

In its essential nature, competition is a decrease in the amount of water and 
light available for each individual, or for each species as represented by the 
total number of individuals. It affects directly these two factors, and through 
them the response of each plant. In a few cases, such as occur when radish 
seeds are planted closely, it is possible to speak of mechanical competition or 
competition for room. The crowding of the swelling roots is, however, only 
an incident in the competition for water, and seems to have no counterpart in 
nature. There is no experimental proof of mechanical competition between 
root-stocks in the soil, and no evidence that their relation is due to anything 
other than competition for the usual soil factors — water, air, and nutrients. 

Competition and dominance. — Properly speaking, competition exists only 
when plants are more or less equal. The relation between host and parasite 
is not competition, nor is that between a dominant tree and a secondary herb 
of the forest floor. The latter has adapted itself to the conditions made by 
the trees, and is in no sense a competitor of the latter. Indeed, as in many 
shade plants, it may be a beneficiary. The case is different, however, when the 
seedlings of the tree find themselves alongside the herbs and drawing upon 
the same supply of water and light. They meet upon more or less equal terms, 
and the process is essentially similar to the competition between seedlings 
alone on the one hand, or herbs on the other. The immediate outcome will 
be determined by the nature of their roots and shoots, and not by the domi- 
nance of the species. Naturally, it is not at all rare that the seedling tree 
succumbs. When it persists, it gains an increasing advantage each succeeding 
year, and the time comes when competition between tree and herb is replaced 
by dominance and subordination. This is the course in every bare area and 
in each stage of the sere which develops upon it. The distinction between com- 
petition and dominance is best seen in the development of a layered forest in 
a secondary area, such as a burn. All the individuals compete with each other 
at first in so far as they form intimate groups. "With the growth of shrubs, 
the latter become dominant over the herbs and are in turn dominated by the 
trees. Herbs still compete with herbs, and shrubs with shrubs, as well as with 
younger individuals of the next higher layer. Within the dominant tree- 
layer, individuals compete with individuals and species with species. Each 
layer exemplifies the rule that plants similar in demands compete when in 



CLEMENTS 






i 



11? 





A. Ecesis in a primary area, summit of Pike's IVak, Colorado. 
B. Ecesis in a secondary area denuded by hot water, Nbrris Geyser Basin, 

Yellowstone Park. 



COMPETITION 73 

the same area, while those with dissimilar demands show the relation of 
dominance and subordination. 

Competition in air and in soil. — The competition between pioneers is usu- 
ally restricted to the soil, where the roots compete with each other for water. 
It is often also the simple competition typical of families, in which all the 
individuals make identical demands because they belong to one species. As 
the families become communities by extension or by migration, the competi- 
tion becomes more complex and the outcome in many cases is dominance. This 
is particularly true as the bare area becomes covered, and success in ecesis 
comes to depend upon the ability to overshadow other plants. The taller 
plant gradually gains the upper hand, partly because it receives more light 
and makes more growth, and partly because its demands are increased by 
greater transpiration. At the same time the shorter plant receives less light, 
grows and transpires less, and its needs for water diminish. This interplay 
of competition and reaction occurs in all communities with individuals of 
different height and extent, but in varying degrees. In pure grassland, com- 
petition of the roots for water is controlling, and the aerial shoots compete 
slightly or not at all. Where broad-leaved herbs play an important or char- 
acteristic part, shoots compete with each other for light. This is true of 
typical prairie to such a degree that actual layers come to be developed, as 
occurs also in other grassland. From the competition in the prairie to that 
of the scrub and the forest is but a change of degree. The dominance of the 
trees is only the outcome of a competition in which position means the control 
of light, and thus of water. Competition of shoots alone may occur when the 
water-supply is in excess, and hence competition for water is absent. This 
is most evident in the case of submerged plants. 

Woodhead (1906) distinguishes communities as competitive when the domi- 
nants occupy the same soil layer, and complementary when the roots are in 
different layers. It is one of the most important tasks of ecology to determine 
the root and shoot relations of communal plants, but it seems much better to 
apply Woodhead 's terms to the species concerned and not to the whole com- 
munity. It is the species which are competitive or complementary, and not 
the community. Moreover, species which are complementary as to roots may 
be competitive as to shoots, and vice versa. In addition, the individuals of 
each species are competing more or less actively, and this is the case with the 
secondary species also, both as to themselves and the dominants. Finally, 
the complementary relation in many cases, if not in all, is merely the outcome 
of the more or less complete success of certain species by which competition 
is changed into dominance. Our knowledge of both competition and domi- 
nance at present is quite too rudimentary to warrant drawing distinctions, 
except as suggestive working hypotheses. 

Role of competition in succession. — As already indicated, competition 
affects the amount of water and of light, even to the point of complete control 
when success in competition becomes dominance; hence its effect upon ecesis 
is direct and often critical. It is seen in the behavior of the seedlings of 
species already in possession, as well as in that of new invaders. Competition 
is most decisive during the development of the seedling and at the time of 
reproduction, particularly in the case of perennials and woody plants. Accord- 
ingly it plays a large part in determining the relative number of occupants 



74 ECESIC CAUSES 

and invaders in each stage of a sere, and thus helps to control the course of 
development. In analyzing the role of competition in the latter, it is desirable 
to distinguish the simple competition of the members of a family and the com- 
petition of the individuals of a single dominant of the primary or other layer 
from the competition between dominant species or that of secondary species. 
As we have seen, the competition between dominant and secondary species 
has ceased and is replaced by a relation of dominance and subordination. 
The reaction of a plant community upon its habitat is largely the sum of the 
habitat effects of competition and dominance. The latter is paramount, how- 
ever, and is chiefly or solely concerned in most important reactions. 

The general effect of competition upon succession has already been indi- 
cated. Its influence may be sketched in some detail by tracing the primary 
development of a spruce forest in brief. The initial crustose lichens which 
colonize the bare rock usually compete with each other little or not at all. 
With the invasion of foliose forms, the competition of the two begins, often 
ending in the complete dominance of foliose Parmelias, etc. The latter 
compete with each other more or less vigorously, even when they occur on 
the rock disintegrated into gravel. Their stabilizing reaction upon the 
gravel-slide aids the invasion of pioneer phanerogams, but there is no compe- 
tition between these and the lichens, even in the case of seedlings. This is 
naturally because of the extreme dissimilarity of their demands. Competi- 
tion appears again only as the result of the slow aggregation of individuals 
into families and colonies, and is rarely if ever an important feature of 
this open stage. With the entrance of a large number of sub-pioneers, the 
number of individuals increases rapidly, and competition for water is often 
acute. The result is that the pioneers disappear rapidly and usually com- 
pletely. The appearance of perennial grasses increases the competition of 
the half-gravel stage, and often translates it into dominance, the resulting 
grassland acting as a subclimax. Often, however, shrubs or aspens enter 
before the grasses become controlling, and the intense competition which 
results passes into a dominance based on light-control. The development of 
the pine stage is regularly conditioned by the reactions of the shrubs. The 
latter and the young pines compete with each other more or less actively for 
a time, but the pines ultimately secure partial dominance at least. When the 
dominance is complete, the pines compete vigorously with each other and 
produce a light reaction unfavorable to the ecesis of their seedlings, but 
favorable to the seedlings of the spruce and fir. The latter succeed in the 
constant competition during seedling and sapling stage, and take their place 
in the primary layer as codominants. The pines decrease in number, probably 
more from the failure of reproduction than from competition with the adult 
spruces and firs. They eventually disappear completely or are represented 
only by an occasional relict. 

While the control of the climax species is now secure except for accidents, 
competition still goes on between the adults as well as the seedlings of each 
year, resulting in oscillations in number. Tt is still a progressive process Avith 
the members of the different layers of the undergrowth as the amount of light 
steadily decreases, and it ceases only with the disappearance of the layers 
caused by the growing absorption of the canopy. During this time, however, 
a secondary effect of competition and dominance is seen in the seasonal aspects 



INVASION 75 

typical of the undergrowth. The appearance of the species of each layer is 
controlled by competition and dominance in such fashion that the layers 
below the dominant one develop in the order of position, the lowermost first, 
before the shrubs have developed their foliage. This effect is of course seen 
most clearly in the aspects of deciduous forests, in which the lowest layer 
consists chiefly or wholly of prevernal or vernal species. A similar and some- 
times equally conspicuous sequence of layers occurs in prairies. 

INVASION. 

Nature and role. — Invasion is the complete or complex process of which 
migration, ecesis, and competition are the essential parts ( Clements, 1904 : 
32; 1905:210; 1907:270). It embraces the whole movement of a plant or 
group of plants from one area into another and their colonization in the 
latter. From the very nature of migration, invasion is going on at all times 
and in all directions. For our purpose it is necessary to distinguish between 
invasion into a bare area and into an existing plant community. The former 
initiates succession, the latter continues the sere by producing successive 
stages until the climax is reached. Invasion does not cease at this point 
necessarily, especially in the presence of artificial processes. As a rule, how- 
ever, invasion into a climax community is either ineffective or it results merely 
in the adoption of the invader into the dominant population. From the 
standpoint of succession only those invasions need be considered which people 
bare areas or produce a new developmental stage. It is obvious that practi- 
cally all invasion in force is of this sort. 

Effective invasion is predominantly local. It operates in mass only between 
bare areas and adjacent communities which contain species capable of pioneer- 
ing, or between contiguous communities which offer somewhat similar condi- 
tions or contain species of wide range of adjustment. Invasion into a remote 
region rarely has any successional effect, as the invaders are too few to make 
headway against the plants in possession or against those much nearer a new 
area. An apparent exception is found in the case of ruderals introduced into 
new countries by man, but these rarely come to be of importance in succession 
until they have been domiciled for many years. The invasions resulting from 
the advance and retreat of the ice during glacial times were essentially local. 
They spread over large areas and moved long distances only as a consequence 
of the advance or withdrawal of the ice. The actual invasion at any one time 
was strictly local. Invasion into a new area or a plant community begins with 
migration when this is followed by ecesis. In new areas, ecesis produces 
reaction at once, and this is followed by aggregation and competition, with 
increasing reaction. In an area already occupied by plants, ecesis and com- 
petition are concomitant and quickly produce reactions. Throughout, the 
development migrants are entering and leaving, and the interactions of the 
various processes come to be complex in the highest degree. 

Kinds of invasion. — Local invasion in force is essentially continuous or 
recurrent. Between contiguous communities it is mutual, unless they are too 
dissimilar. The result is a transition area or ecotone which epitomizes the 
next stage in development. By far the greater amount of invasion into exist- 
ing vegetation is of this sort. The movement into a bare area is likewise 
continuous, though it is necessarily not mutual, and hence there is no ecotone 



76 ECESIC CAUSES 

during the earlier stages. The significant feature of continuous invasion is 
that an outpost may be repeatedly reinforced, permitting rapid aggregation 
and ecesis, and the production of new centers from which the species may be 
extended over a wide area. Contrasted with continuous invasion is inter- 
mittent or periodic movement into distant regions, but this is rarely con- 
cerned in succession. When the movement of invaders into a community is 
so great that the original occupants are driven out the invasion is complete. 
This is characteristic of the major stages of succession, though there are neces- 
sarily transitions between these, often of such character as to require recogni- 
tion. Major stages, and especially subclimaxes and climaxes, often undergo 
partial invasion without being essentially changed. While the permanence 
of invasion varies greatly, the terms "permanent" and "temporary" are 
purely relative. In each sere initial and medial stages are temporary in 
comparison with the climax. The initial stages of a primary sere may last 
for centuries, but they must finally pass in the course of development. Climax 
stages are permanent, except in the case of destruction or an efficient change 
of climate. In the geological sense, however, they are transient stages of 
the geosere. 

Manner of invasion. — Bare areas present very different conditions for in- 
vaders to those found in plant communities. This is due to the absence of 
competition and often of reaction. Conditions for germination are regularly 
more favorable in plant communities, but the fate of seedling and adult is 
then largely determined by competition. Open communities are invaded 
readily, closed ones only with difficulty, if at all. It is important to recognize 
that a community is not necessarily open because part of the surface is bare. 
Secondary bare areas usually afford maximum opportunity for invasion. This 
is due partly to the lack of competition, but especially to the fact that condi- 
tions are more or less optimum for the germination and growth of a wide 
range of species. Primary areas, on the contrary, present only extremes of 
water-content, and thus exclude all invaders except a few pioneers. 

Invasion into a bare area may be lateral, peripheral, or general. It is 
lateral in all land areas bordered by deep water, since successful invaders can 
reach it only from land communities. It may be bilateral when the water is 
shallow enough to contain amphibious species and the area sufficiently wide to 
permit a gradual change of conditions. When the bare area lies between two 
different terrestrial associations the movement is regularly from both direc- 
tions, if conditions are not too extreme. If it is surrounded by an association 
or associes, but particularly the former, the invasion takes place all along 
the edge. When the area is large the invaders move forward into it by re- 
peated advances, often producing temporary zones. In small areas such a 
zonal invasion is typical when species invade by propagules. In many secon- 
dary areas, especially burns and abandoned fields, the migration is general, 
and the area is more or less completely covered in the initial stage. 

In all invasions after the first or pioneer stage, the relative level of occu- 
pants and invaders is critical. A community may be invaded at its level, 
i. e., by species of the same general height as those in occupation, or below or 
above this level. When invasion is at the same height, the level has no effect 
and the sequence is determined by other features. If it is above the level of 
the occupants, the newcomers become dominant as they stretch above their 



INVASION 77 

neighbors and soon give character to a new stage. This is typically the case 
with shrubs and trees, in which the close dependence of the sequence of stages 
upon life-form is most evident. When invasion is below the existing level, it 
has no direct influence upon the dominant species. Such invaders normally 
take a subordinate place as secondary components of the community. In rare 
instances they play an important or decisive part by virtue of some advan- 
tageous competition form, such as the rosette or mat, or of some unique 
reaction, as in Sphagnum. 

Barriers. — A topographic feature or a physical or a biological agency that 
restricts or prevents invasions is a barrier. Topographic features are usually 
permanent and produce permanent barriers. Biological ones are often tem- 
porary and exist for a few years or even a single season. Temporary barriers 
are often recurrent, however. Barriers are complete or incomplete with 
respect to the thoroughness of their action. They may affect invasion either 
by limiting migration or by preventing ecesis. It has been generally assumed 
that their chief effect is exerted upon migration, but it seems clear that this 
is not the case. Even in the case of extensive barriers, such as the ocean, the 
influence upon ecesis is decisive. 

Barriers are physical when due to some marked topographic feature, such 
as an ocean, lake, river, mountain range, etc. All of these are effective by 
virtue of their dominant physical factors. They prevent the ecesis of the 
species coming from very different habitats, though they may at the same 
time serve as conductors for plants from similar habitats. This is especially 
true of water-currents and mountain ranges. A body of water with its exces- 
sive water-content is a barrier to mesophytes and xerophytes, but a conductor 
for hydrophytes. Deserts set a limit to the invasion of mesophytic and hydro- 
phytic species, while they favor that of xerophytes. By its reduction of tem- 
perature, a high mountain range restricts the extension of plants of lowlands 
and plains. It is also more of an obstacle to migration than most physical 
barriers, because of the difficulty of movement up its slopes. Any bare area 
with extreme conditions is a barrier to the invasion of communities beyond. 
It is not to be regarded as a barrier to the development of succession upon it, 
since the proper pioneeers are always able to invade it. 

Biological barriers. — Biological barriers comprise plant communities, man 
and animals, and parasitic plants. The limiting effect of a plant community 
is exhibited in two ways. In the first place, an association acts as a barrier 
to the ecesis of species invading it from associations of another type, on 
account of the physical differences of the habitats. Whether such a barrier 
be complete or partial will depend upon the relative unlikeness of the two 
areas. Shade plants are unable to invade a prairie, though the species of open 
thickets or woodland may do so to a certain degree. A forest formation, on 
account of its diffused light, is a barrier to poophytes, while a swamp, because 
of the amount and kind of water-content, sets a limit to the species of both 
woodland and grassland. Such formations as forests and thickets act also as 
direct obstacles to migration in the case of tumbleweeds and other anemo- 
chores, clitochores, etc. Closed communities likewise exert a marked influence 
in decreasing invasion by reason of the intense and successful competition 
which all invaders must meet. Closed associations usually act as complete 
barriers, while more open ones restrict invasion in direct proportion to the 



78 ECESIC CAUSES 

degree of occupation. To this fact may be traced the fundamental law of 
succession that the number of stages is determined largely by the increasing 
difficulty of invasion as the area becomes stabilized. Man and animals affect 
invasion by the destruction of germules. Both in bare areas and in serai 
stages the action of rodents and birds is often decisive to the extent of altering 
the whole course of development. Man and animals operate as marked 
barriers to ecesis wherever they alter conditions unfavorably to invaders or 
where they turn the scale in competition by cultivation, grazing, camping, 
parasitism, etc. The absence of pollinating insects is sometimes a curious 
barrier to the complete ecesis of species far out of their usual habitat or 
region. Parasitic fungi decrease migration in so far as they affect seed 
production. They restrict or prevent ecesis either by the destruction of 
invaders or by placing them at a disadvantage with respect to the occupants. 
Changes in barriers. — A closed formation, such as a forest or meadow 
which acts as a decided barrier to invasion, may disappear completely as the 
result of a land-slide, flood, or burn, and leave an area into which invaders 
crowd from every point. A temporary swing of climate may disturb the 
balance of a community so that it permits the entrance of mesophytes which 
are normally barred, and one or more stages of succession may be omitted as 
a consequence. On the other hand, a meadow or swamp, for example, ceases 
to be a barrier to prairie xerophytes during a period of unusually dry years, 
such as regularly occurs in semiarid regions. A peculiar example of the 
modification of a barrier is afforded by the complete defoliation of aspen 
forests in the Rocky Mountains. As a result, they were invaded by poophytes, 
producing a change of development identical with that found in the usual 
aspen clearing. Nearly all xerophytic stretches of sand and gravel, dunes, 
blow-outs, gravel-slides, etc., as well as prairies and plains in some degree, 
exhibit a recurrent seasonal change in the spring. As a result, the dry, hot 
surface becomes sufficiently moist to permit the germination and growth of 
invaders, which are normally barred out during the rest of the year. The 
influence of distance as a barrier has already been indicated under 
"Migration." 



V. REACTIONS. 

Concept and nature. — By the term reaction is understood the effect which 
a plant or a community exerts upon its habitat (Clements, 1904:124; 1905: 
256; 1907:282). In connection with succession, the term is restricted to this 
special sense alone. It is entirely distinct from the response of the plant or 
group, i. e., its adjustment and adaptation to the habitat. In short, the 
habitat causes the plant to function and grow, and the plant then reacts upon 
the habitat, changing one or more of its factors in decisive or appreciable 
degree. The two processes are mutually complementary and often interact in 
most complex fashion. As a rule, there is a primary reaction with several or 
many secondary ones, direct or indirect, but frequently two or more factors 
are affected directly and critically. Direct reactions of importance are con- 
fined almost wholly to physical factors, with the exception of parasitism, 
which can hardly be regarded as a reaction proper. With almost no excep- 
tions, reactions upon biological factors have barely been touched by investi- 
gators as yet. Any exact understanding of them must await the quantitative 
study of the community as a biological unit. 

The reaction of a community is regularly more than the sum of the reactions 
of the component species and individuals. It is the individual plant which 
produces the reaction, though the latter usually becomes recognizable through 
the combined action of the group. In most cases the action of the group 
accumulates or emphasizes an effect which would otherwise be insignificant 
or temporary. A community of trees casts less shade than the same number 
of isolated individuals, but the shade is constant and continuous, and hence 
controlling. The significance of the community reaction is especially well 
shown in the case of leaf-mold and duff. The leaf-litter is again only the total 
of the fallen leaves of all the individuals, but its formation is completely 
dependent upon the community. The reaction of plants upon wind-borne 
sand and silt-laden waters illustrates the same fact. 

Some reactions are the direct consequence of a functional response on the 
part of a plant. This is exemplified by the decrease of water-content by 
absorption, the increase of humidity as a consequence of transpiration, and 
the weathering of rock by the excretion of carbon dioxid. Others are the 
immediate outcome of the form or habit of the plant body. The difference 
between woody plants and grasses in the reaction upon light and humidity is 
one of the critical facts in succession. Almost any obstruction may cause 
the deposition of dune-sand or of water-borne detritus. The actual formation 
of a dune depends, however, upon the aerial and soil forms so typical of sand- 
binders. The accumulation of leaf-mold, filling with plant remains, and the 
production of humus are all due to the death and decay of plants and plant 
parts. Marl, travertine, calcareous tufa, and sinter are partly or wholly the 
result of little-understood processes of the plant. The successful reaction of 
pioneers in gravel-slides and in bad lands is almost wholly a matter of mat, 
rosette, or bunch forms and of extensive or deep-seated roots. In a primary 
area the reaction is exerted by each pioneer alone, and is then augmented by 
the family or colony. It extends as the communities increase in size, and 

79 



80 REACTIONS 

comes to cover the whole area as vegetation becomes closed. It is often felt 
for a considerable space around the individual or group, especially when 
exerted against the eroding action of wind or water, or the slipping conse- 
quent upon gravity. In most secondary areas and serai stages the reaction is 
the combined effect of the total population. In it the preponderant role is 
played by successful competitors and particularly by the dominants. These 
determine the major or primary reactions, in which the part of the secondary 
species is slight or negligible. 

Role in succession. — In the development of a primary sere, reaction begins 
only after the ecesis of the first pioneers, and is narrowly localized about them 
and the resulting families and colonies. It is necessarily mechanical at first, 
at least in large degree, and results in binding sand or gravel, producing 
finely weathered material, or building soil in water areas, etc. In secondary 
seres, extensive colonization often occurs during the first year and reaction 
may at once be set up throughout the entire area. The reactions of the 
pioneer stage may be unfavorable to the pioneers themselves, or they may 
merely produce conditions favorable for new invaders which succeed grad- 
ually in the course of competition, or become dominant and produce a new 
reaction unfavorable to the pioneeers. Naturally, both causes may and often 
do operate at the same time. The general procedure is essentially the same 
for each successive stage. Ultimately, however, a time comes when the reac- 
tions are more favorable to occupants than to invaders, and the existing com- 
munity becomes more or less permanent, constituting a climax or subclimax. 
In short, a climax vegetation is completely dominant, its reactions being such 
as to exclude all other species. In one sense, succession is only a series of 
progressive reactions by which communities are selected out in such a way 
that only that one survives which is in entire harmony with the climate. 
Reaction is thus the keynote to all succession, for it furnishes the explanation 
of the orderly progression by stages and the increasing stabilization which 
produces a final climax. 

Previous analyses of reaction. — The essential nature of reaction has been 
little recognized in the past, and there have been but two attempts to analyze 
and group the various reactions. Clements (1904:124; 1905:257; 1907:282) 
pointed out that the direction of movement in succession was the immediate 
result of its reaction, and that the latter is expressed chiefly in terms of water- 
content. He further stated that the initial causes of succession must be sought 
in the physical changes of the habitat, but that the continuance of succession 
depended upon the reaction which each stage exerted upon the habitat. The 
general reactions of vegetation were classified as follows: (1) preventing 
weathering, (2) binding aeolian soils, (3) reducing run-off and preventing 
erosion, (4) filling with silt and plant remains, (5) enriching the soil, (6) ex- 
hausting the soil, (7) accumulating humus, (8) modifying atmospheric factors, 
light, humidity, etc. Cowles (1911 :173) has classified plant and animal agencies 
in succession in five groups: (1) humus complex, (a) water, (&) soil organisms, 
(c) toxicity, (d) food, (e) temperature and aeration; (2) shade; (3) plant 
invasion; (4) man; (5) plant plasticity. The factors of the humus complex 
and shade are reactions, as the term is understood here. Invasion is the basic 
process of which succession is but the continuance or recurrence ; man is an 
initial cause, and plasticity a response to the habitat as modified by reaction. 



SOIL FORMATION 81 

Kinds of reactions. — Since two or more major reactions regularly occur in 
a primary sere, and in many secondary ones also, it is impossible to classify 
them on a strictly developmental basis. It is most convenient to group them 
in accordance with their nature and effect, an arrangement which is likewise 
fundamental because it emphasizes the directive influence of reactions. While 
it is helpful to distinguish them as primary and secondary with respect to a 
particular sere, such a general distinction is not feasible, owing to the fact that 
a reaction may be primary in one sere and secondary in another, or in differ- 
ent periods of the same sere. The main division may well be made upon the 
seat of the reaction, which results in the two groups, (1) soil reactions and (2) 
air reactions. The soil as a fixed substratum is much more affected by plants, 
and the soil reactions are correspondingly much more numerous than those in 
the air. They do not permit of any precise subdivision, since soil factors are 
so intimately related. It is helpful in permitting a comprehensive view to 
group them in accordance with the factor directly affected. This results in the 
following arrangement: (1) soil formation and structure, (2) water-content, 
(3) solutes, (4) soil organisms. The subdivision of air reactions is less satis- 
factory, but the following will serve our present purpose: (1) light; (2) other 
factors (humidity, etc.) ; (3) aerial organisms. 

In the following discussion of reactions in detail, an endeavor is made to 
indicate the cause of each reaction, to trace its effect upon the habitat, and to 
relate this to the development of the succession. Some of the recent quantita- 
tive studies of reactions are also indicated. The exact study of this most 
difficult portion of the field of succession has barely begun, and the many gaps 
in our knowledge are consequently not surprising. 

SOIL FORMATION. 

Manner. — The reactions of plants upon the substratum fall into two cate- 
gories, viz, (1) those which produce a new substratum or soil and (2) those 
which affect and usually change the texture of the soil. 

A new substratum may be formed in four essentially different ways : (1) by 
the accumulation of the plant bodies themselves, usually under conditions 
which retard or prevent decay; (2) by the concretion of mineral matters into 
rock or marl through the activity of water plants; (3) by the weathering of 
rock into fine soil by the excretion of acids; (4) by the resistance which plant 
bodies offer to moving air and water, resulting in the deposition of particles in 
transport. Plants modify the structure of the soil primarily as a result of the 
death and decay of plant bodies and parts, a reaction differing from the accu- 
mulation of plant remains into a new soil only in the degree of accumulation 
and of decaj\ They also affect soil-texture in consequence of the penetration 
of their roots and the accompanying liberation of carbon dioxid. but this effect 
hardly seems a significant one. The most striking reaction upon soil-structure 
occurs in the formation of a rocky layer termed "ortstein" from the typical 
"bleisand" of many heaths. Another group of reactions affect the soil by 
preventing weathering, or the erosion of the surface by wind and water. 

(1) Reaction by accumulating plant bodies or parts. — The complete decom- 
position of plants in contact with air prevents any considerable heaping-up 
of plant remains in ordinary habitats. Accumulation in quantity can occur 
in consequence only under water, where oxidation is largely or completely 

7 



82 v REACTIONS 

prevented. This is the universal method by which biogenous soils are formed, 
thought it must be recognized that animals also usually play a large or con- 
trolling part in the process. As a reaction proper it is brought about only by 
plants which grow in water or in wet places, but the formation of the soil may 
be hastened by the incorporation of transported material, including terrestrial 
plants as well as animal remains and detritus. It is the characteristic reaction 
of aquatic and amphibious communities, and occurs in salt water as well as in 
fresh water. The peat substratum which results is found universally wherever 
plants decompose in the presence of insufficient oxygen. As is well known, a 
similar process has recurred throughout geological history, resulting in the 
formation of coal at various times from the Paleozoic to the Tertiary. Along 
with the biogenous formation of the soil occur certain secondary consequences, 
such as the production of acids, of possible toxic substances, changes in soil 
organisms, etc., which are considered elsewhere. 

The shallowing of water by pioneer aquatics first changes the conditions to 
the detriment of submerged plants and the advantage of floating species, and 
then to the respective disadvantage and advantage of floating and amphibious 
plants. This is equally true when water-borne detritus plays a part, for it 
merely hastens the outcome. The process is continued by the amphibious reeds 
and sedges, which may yield finally to meadow grasses. In this stage, surface- 
water usually disappears, and the accumulation ceases entirely or nearly so, 
because of the access of oxygen. In boreal and mountain regions Sphagnum 
usually enters in the amphibious stage or near its close, and gives a new lease 
to accumulation under circumstances which may almost completely inhibit 
decomposition. After a time the moss layer becomes so thick that other plants 
may enter because of the decreasing water-content of the surface, which con- 
trols the further development. Sphagnum may also extend as a floating mat 
over pools and ponds, and eventually fill them with peat (plate 10 a). 

A host of investigators have studied the formation of soil by peat-producing 
plants (Plant Succession, pp. 238, 378). Various kinds of peat have been dis- 
tinguished on the basis of the component species and the degree of decompo- 
sition and compression. These have little bearing on the reaction here con- 
sidered, since the mere accumulation is the chief fact. The direct reaction 
which influences the sequence of stages is, however, the change in water depth 
and content incident to the increase of thickness of the peat. In the sub- 
merged and floating stages the directive factor is the decreasing depth which 
permits the entrance of species with floating leaves. Such plants cut off the 
light from the submerged pioneers and probably change other conditions 
unfavorably also. A further reduction of depth allows the eeesis of amphibi- 
ous reeds, and these first dominate and then displace the floating plants, 
partly, it seems, in consequence of light reduction. From this point the essen- 
tial change is a decrease of water-content, largely by continued filling but 
partly because of the relative increase in transpiration. This is the ruling 
reaction throughout the rest of the development, unless the latter is deflected 
by the appearance of Sphagnum, or until it reaches the shrub or forest stage. 

The formation of soil by the deposition of diatom shells is relatively insig- 
nificant, though frequently found on a small scale. It probably played a 
larger part in the geological past, if one may judge from the existence of exten- 
sive diatom beds in various places in Nebraska, California, Nevada, etc. 



SOIL FORMATION 83 

While the production of diatomaceous soil may be seen along the margins of 
many pools and streams, diatom marshes of large extent are rare. Weed 
(1887) has traced their development in Yellowstone Park, and has found that 
extensive meadows have been built up in this way. In spite of the difference of 
material and the absence of certain secondary influences, the primary reaction 
has to do with the decrease in the amount of water, as in the case of peat 
areas. 

(2) Reaction by accumulating plant concretions. — The rocky substrata due 
to the direct physiological activity of plants are either calcareous or siliceous, 
the former being much more common. Calcareous substrata are represented 
by marl, travertine, calcareous tufa and perhaps by oolite; siliceous ones by 
sinter or geyserite. Concretions of either sort are usually formed by alga? 
and are especially characteristic of hot springs. Aquatic mosses also possess 
the power of secreting travertine and tufa. Chara plays the chief role in the 
formation of marl (Davis, 1900, 1901), while Rothpletz (1892) assumes that 
oolite is due to the calcareous secretions of a blue-green alga. Cohn (1862) 
was the first to point out the connection of algae with the formation of tufa 
and sinter. The first studies of importance in this country were made by Weed 
(1889) in Yellowstone Park, and these have been supplemented by those of 
Tilden (1897, 1898) in the Rocky Mountain region generally. Tilden has 
described 24 algae from the hot springs of this region, and it is probable that 
all of these play a part in rock formation. The yellow-green alg* (Chloro- 
phyceae) are represented by Oedogonium, Hormiscia, Conferva, Microspora, 
Rhizoclonium, and Protococcus. The blue-green thermal algje (Cyanophyceae) 
belong to the genera Calothrix, Rivularia, Ilapalosiphon, Schizothrix, Sym- 
ploca, Phormidium, Oscillatoria, Spirulina, Synechococcus, Gloeocapsa, and 
Chroococcus. In the case of the marl or lime deposit of lakes, Davis finds that 
it is made up of coarser and finer material derived from the incrustations on 
Schizothrix and Chara, but principally the latter. 

From the standpoint of succession, concretion into solid rock is very differ- 
ent from that by which marl is produced. The compactness of travertine, 
sinter, and oolite is doubtless due to the microscopical size of the alga? con- 
cerned. In the case of marl formed largely by Chara, the stem and leaves of 
the latter are so large relatively that their death and decay breaks up the con- 
cretions in large degree. The fragile branching stems and leaves also prevent 
compacting into a solid mass. Marl, moreover, accumulates in ponds and 
lakes, where its action is to shallow the water and to produce much the same 
results already noted for peat and diatom soils. In fact, the action is essen- 
tially identical so far as the initiation of the water sere and the direction of the 
first stages are concerned. Sinter and travertine are formed locally as super- 
ficial deposits under conditions which are unfavorable to colonization, though 
this does begin at the edges of the cooler brooks which drain the hot-spring 
areas. The essential fact, however, is that they are biogenic rocks and can 
only form initial areas for primary succession, instead of directing the sequence 
of stages. As in the case of tufa and oolite, the reaction of the concretionary 
algas leads to the origin of a new rock sere, while in the formation of marl by 
Chara it continues and directs a water sere already begun (plate 22a). 

(3) Reaction by producing iveathering. — The primary reaction of plants 
upon rocks is the decomposition of the surface into an exceedingly fine soil. 



84 REACTIONS 

A secondary influence is the production and widening of cracks by means of 
roots and stems, but this is often lost sight of in the greater effects of atmos- 
pheric weathering. It is also impossible in many cases to separate the effect of 
plants and atmosphere in the intimate decomposition of rock surfaces. As a 
rule, however, the paramount action of the plants is indicated by its localiza- 
tion upon certain surfaces or areas. All pioneers on rocks break down the 
surface in consequence of their excretion of carbon dioxid or other acids, and 
produce a fine layer of dust. In the case of lichens and many mosses this layer 
remains in place, but usually it is carried into cracks and crevices. This slow 
production of a thin soil or shallow pocket is reinforced by the decay of the 
pioneers themselves, which also materially increases the nutrient-content and 
the water-holding capacity. Here, again, it is almost impossible to separate 
the two reactions; but this is immaterial, since their effect is the same. The 
combined effect is to produce areas in which rock herbs can secure a foothold 
and to increase slowly the water-content and the nutrient-content. 

The reaction through weathering takes place most readily when the rock is 
sedimentary and soft, especially if it is wet or moist during a large part of the 
growing-season. In such places, the pioneers are mostly mosses and liver- 
worts, often preceded by algae. Lichens are much less frequent and are apt 
to be collemaceous. Water is abundant, and the effect is chiefly to produce 
a foothold for herbs, apart from the increase of humus. As a consequence, 
the pioneer stages are often extremely short, and the rocky surface may be 
quickly covered with herbaceous or even shrubby vegetation. When the 
rock is exposed to wind and sun, and especially when it is igneous, biogenous 
weathering begins with the crustose lichens. The influence is exerted at the 
contact of thallus and rock, but the corroding carbon dioxid and other secre- 
tions act also beyond the margin of the thallus during moist periods. This 
permits the slow extension of each thallus and the starting of new ones, with 
the result that the rock surfaces with upward or north to east exposure become 
completely incrusted. The centers of the older thalli sooner or later die and 
begin to break up, leaving an area of greater water-retaining capacity for the 
invasion of f oliose lichens. By their greater size and vigor these extend more 
rapidly, gradually covering the crustose species and causing them to die as a 
result of the decrease of water and of light. The size and thickness of the 
foliose thallus enable it to retain water better, and thus to enhance its power 
to weather the surface to greater depths. The surface is usually rough and 
uneven by reason of folds, soredia, etc., and this helps materially in retaining 
the water, as well as in providing lodging-places for the spores of mosses. In 
their turn the foliose thalli break up at the center and offer a favorable field 
for the invasion of mosses and, more rarely, of low, matlike herbs. In the 
weathering of the granites and other hard rocks of the Rocky Mountains such 
herbs follow the mosses and form the fourth stage. In both stages the amount 
of soil steadily increases, and with it the amount of water. The disappearance 
of the mosses is apparently due to the change of light intensity and to the root 
competition of the herbs. The herbaceous mats form almost ideal areas for 
the colonization of large herbs and grasses, especially at the center, where 
they first die and decay (plate 10b). 

(4) Reaction upon wind-borne material. — This is the reaction which results 
in the formation of dunes and sand-hills, and probably also of deposits of 



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H. Reaction by causing weathering, Pilot Knob, Pike's Peak, Colorado. 



SOIL FORMATION 85 

loess. It is the outcome of the retardation of air-currents hy the stems and 
leaves of plants, especially pioneers in sand. The effect of the plant-body 
is twofold ; it is not only a direct obstacle to the passage of grains of sand, but 
it also decreases the velocity of the wind and hastens the consequent dropping 
of its load. The same action likewise tends to prevent the wind from picking 
the sand up again and carrying it further. The underground parts of sand 
plants exert a complementary reaction by binding the sand through the 
action of roots and rhizones, and by developing shoots which keep pace with 
the rise of the surface. Certain pioneers form rosettes or mats, which hold 
the sand with such firmness that they cause the formation of hummocks with 
a height of one to many feet above the bare areas. The behavior of sand- 
binders has been a fruitful field of study, and there is probably no other group 
of plants whose reactions are so well understood (plate 1a). 

The primary reaction upon wind-blown sand is mechanical. The pioneer 
grasses in particular stop and fix the sand and produce stable centers for 
invasion. This permits the entrance of other species capable of growing in 
bare sand, if it is not shifting actively. With the increase of individuals, how- 
ever, the amount of vegetable material in the soil becomes greater, increasing 
the water-retention of the sand and the amount of nutrients. This is the prim- 
ary reaction in sand areas after the sand-binders have finished their work of 
stabilization. The reaction which produced and colonized deposits of loess 
must have been similar. The action of plants in bringing about the dropping 
and temporary fixing of wind-blown dust must indeed have been almost identi- 
cal. Because of their much smaller size the dust particles were much more 
readily compacted by the action of rainfall. For the same reason they 
retained more of the latter in the form of the holard, and loess areas were 
probably xerophytic for a much shorter time. While the development of 
the first stages was doubtless more rapid, each stage necessarily increased the 
humus and hence the water-content, though to a less significant degree perhaps 
than in sand. However, our knowledge of the initial stages on loess and of 
their reaction is obtained mostly from analogy, since no deposits of loess 
known to be forming at the present time have been studied critically (cf. 
Shimek, 1908:57; Huntington, 1914 2 :575). 

(5) Reaction upon water-borne detritus. — The effect of plant bodies upon 
material carried by water is essentially similar to that noted for eolian sand. 
Stems and leaves slow the current and cause the deposition of its load in whole 
or in part (plate 4). They also make difficult the removal of material once 
deposited, a task in which roots and root-stocks have a share likewise. This 
reaction is often associated with the deposition of sand and silt by the retarda- 
tion of currents as they empty into bodies of water, but the effect of plants is 
usually predominant. The filling incident to this reaction has the conse- 
quences already indicated for filling by the accumulation of plant remains. 
In fact, both processes cooperate to decrease the depth of water wherever 
plants occur in an area through which detritus is carried. The decreasing 
depth controls the usual sequence from submerged to amphibious plants. 
The latter continue the process, but the movement, of the water is steadily 
impeded as the level rises, until finally it overflows the area only at times of 
flood. This sets a limit to the accumulation of detritus, and the further 
development is controlled by decreasing water-content due to plant aecumula- 



86 REACTIONS 

tions, to transpiration, etc. Frequently the deposit of silt and subsequent 
heaping-up of plant materials go on more rapidly in some spots than in others, 
producing hummocks on which the future course of development is traced in 
miniature. 

(.6) Reaction upon slipping sand and gravel. — A characteristic feature of 
the Rocky Mountains is the steep talus-slope known as a gravel-slide. The 
angle of the slope is usually so great that some slipping is going on constantly, 
while the movement downward is materially increased after a heavy rain. 
The fixation of such a slope is a problem similar to that which occurs in dunes 
and blow-outs. The coarse sand or gravel must be stopped and held in opposi- 
tion to the downward pull due to gravity. The movement is slower and is 
somewhat deeper-seated. Consequently, the species best adapted to gravel- 
slides are mats or rosettes with tap-roots or long, branching roots. The latter 
anchor the plant firmly and the cluster of stems or horizontally appressed 
leaves prevents the slipping of the surface area. Each plant or each colony 
exerts the stabilizing effect for some distance below its own area, owing to the 
fact that it intercepts small slides that start above it. The primary reaction 
is a mechanical one, and a large number of species invade as soon as the sur- 
face is stable. These increase the humus production and water-content, and 
the subsequent reaction resembles that of all dry sand or gravel areas (plate 2a) . 

SOIL-STRUCTURE. 

The structure of the soil may be changed mechanically by plants through 
the admixture of plant remains, the penetration of roots, or the compacting 
incident to the presence of plants. Associated with these are chemical changes 
often of the most fundamental importance. In addition, plants react upon 
the soil in such a way as to protect it against the action of modifying forces, 
such as weathering and erosion by water or wind. None of these are simple 
reactions, but the mechanical effect of each may constitute a primary reaction. 
The opportunity for greater clearness and analysis seems likewise to warrant 
the consideration of their influence upon the basis of soil structure and profile. 

(7) Reaction by adding humus. — The change in the texture of the soil due 
to the admixture of humus is caused by animals as well as by plants. In 
grassland and woodland soil, animals indeed play the chief part in the dis- 
tribution of humus in the soil. The effect of the humus is much the same, 
however, quite apart from the fact that soil organisms work over only material 
which is destined to become humus at all events. All plant communities pro- 
duce humus in some degree by the death of entire plants, annually or from 
time to time, and by the annual fall of leaves and the aerial parts of perennial 
herbs. The amount produced depends upon the density and size of the popu- 
lation and upon the rate and completeness of decomposition. It is small in 
the pioneer stages of a sere, especially in xerophytic situations, and increases 
with each succeeding stage. It reaches a maximum in mesophytic grassland 
and woodland, but falls off again with the decrease of population in a com- 
pletely closed community. 

The physical effect of humus is to make light soil more retentive of water 
and heavy soils more porous. Hall (1908:47) states this as follows: "Humus 
acts as a weak cement and holds together the particles of soil ; thus it serves 
both to bind a coarse-grained sandy soil, and, by forming aggregates of the 



SOIL-STRUCTURE 87 

finest particles, to render the texture of a clay soil more open. ' ' In general, 
it increases the water-content of dry, bare areas and tends to decrease the 
water-content of moist areas. The latter is chiefly the result of raising the 
level, and is often complicated by decreasing aeration and the possible produc- 
tion of harmful substances through partial decomposition. The effect of 
humus is most marked in the weathering of rock and in dry sand and gravel 
areas, where the action is cumulative throughout the whole course of develop- 
ment. The increase in the number or size of the individuals in each successive 
stage results in more material for humus production, and this increases the 
water-content steadily from the initial to the climax stage. While the holard 
increases, the echard also mounts from less than 1 per cent in sand and 
gravel to 12 to 15 per cent in loam, so that the chresard increases less rapidly 
than the total water-content. The ultimate effect in each stage is to favor the 
invasion of plants with greater water requirements, and hence with greater 
powers of competition and duration. They readily become dominant and their 
predecessors disappear or become subordinate. 

The penetration of roots tends to make hard soil looser in texture and to 
increase the available water, while it decreases the permeability of sand and 
raises the holard correspondingly. It is so intimately associated with humus 
in its effects that it is difficult if not impossible to distinguish between them. 

(8) Reaction by compacting the soil. — This is an indirect effect due to the 
reaction of the community upon the water-content. It constitutes a reaction 
of primary importance in the case of heath on sandy soils, and perhaps also 
in the "hard" lands of the Great Plains. In heath-sand the final outcome is 
the formation of a rock-like layer at a depth of 2.5 to 3 dm. This is the layer 
known as ' ' ortstein. ' ' There is still much doubt as to the process by which it 
is formed, and it seems probable that it may arise in different ways. Graebner 
(1909) assumed the usual formation of "ortstein" to be as follows: 

The humus substances characteristic of heath-sand remain in solution only 
in pure or in acid water, but are precipitated in the presence of the soil salts. 
They pass through the heath-sand almost unchanged, but are precipitated 
where the sand lies in contact with a substratum richer in mineral salts. Here 
is formed a brown layer which further accumulations of humus precipitates 
convert into the true "ortstein" which may reach a decimeter in thickness. 
The primary effect of "ortstein" is mechanical in that it stops the downward 
growth of roots completely. It seems to have an influence apart from this 
also, inasmuch as roots grow poorly even when they pass through openings in 
the layer. The horizontal growth of roots is also found where the layer is 
not sufficiently compact to prevent penetration. This effect seems to be due 
to poor aeration caused by a lack of oxygen. 

The effect of "ortstein" upon the course of succession is to handicap deep- 
rooted plants, such as shrubs and trees, and to retard or prevent the appear- 
ance of the final stages. Instead of producing or favoring the progression of 
stages, as most reactions do, it limits development and tends to make the 
heath the climax association. A somewhat similar result occurs in grassland 
communities in arid or semiarid regions, where the penetration of water is 
limited to the root layer. The soil beneath becomes densely compacted into 
a layer known as "hardpan." As a result, deeper-rooted species are elim- 
inated and the area comes to be dominated by the characteristic "short- 



88 REACTIONS 

grasses" (Shantz, 1911). Both "hardpan" and "ortstein" favor the per- 
sistence of the community which produced them. "Hardpan," however, 
brings about the disappearance of the preceding population, while "ortstein" 
apparently does not appear until heath has long been in possession, since it 
depends upon the production of heath-sand. Another difference lies in the 
fact that heath is at most a subclimax, while the "short-grass" association 
is the final climatic stage (now shown to be subclimax; cf. p. 183). 

(9) Reaction by preventing weathering or erosion. — A plant cover, whether 
living or dead, everywhere produces an important reaction by protecting the 
surface from erosion. It has a somewhat similar effect upon the weathering 
of rock by atmospheric agents, but this has much less significance, since the 
plants themselves are producing weathering. In the case of erosion, the 
reaction is much the same as that which occurs when plants stop drifting 
sand or suspended silt. In open communities the stems and leaves reduce 
the velocity of wind or water and make it difficult for them to pick up soil 
particles; in closed associations the plants usually eliminate the effect of 
wind and water entirely and the erosion is null. The influence of cover is 
thus a progressive one, from the sparse population of the pioneer stage with 
most of the surface exposed to erosive action, through more and more closed 
communities to the climax. It is a stabilizing factor of the first importance 
in that it prevents denudation and consequent initiation of a new area. At 
the same time it assures continued occupation by the plants in possession, and 
hence the continuance of the reactions which produce the normal sequence of 
stages. The progressive increase of reaction tends to limit denudation and 
the renewal of succession largely to the early stages, and makes it more diffi- 
cult in the final ones. Its significance is of course clearly revealed when the 
cover is partially or wholly destroyed (plate 11, a, b). 

WATER-CONTENT. 

Since water is the chief factor in succession, as in plant response, it is more 
or less affected by practically all reactions. In addition, the increase or 
decrease of water-content may be the direct outcome of the activity of the 
plant itself. The effect, moreover, may be exerted on the chresard as well as 
upon the total water-content. 

(10) Reaction by increasing water-content. — There seems to be no case in 
which flowering plants increase water-content as a direct reaction. Their 
influence in reducing loss by evaporation from the soil is really due to the 
effect of shading. In the case of Sphagnum, however, the power of the plant 
to absorb and retain large amounts of rain and dew is a direct reaction of 
primary importance. Because of this property, Sphagnum is able to waterlog 
or flood an area and to deflect the sere or initiate a new one. In the moss areas 
themselves the effect is essentially to produce a new area of excessive water- 
content, which can be invaded only as the surface becomes drier. The ability 
of Sphagnum to retain water, either when living or in the form of peat, is 
also a controlling factor in the course of the development of the new sere. 

The accumulation of plant remains as humus is the universal process by 
which the amount of water-content is increased. No plant community fails 
to produce humus in some degree ; hence no soil escapes its action, though this 
is often inconsiderable in the initial stage of xerophytic areas. Its influence 



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A. Reaction by preventing weathering, crustose lichens, Picture Rocks, Tucson, Arizona. 

B. Consocies of Chrysothamnus reducing water erosion in marginal gullies of bad lands, 

Scott's Bluff, Nebraska. 



NUTRIENTS AND SOLUTES 89 

is best seen in sand and gravel, where the addition of a small amount of 
humus greatly increases the water-holding capacity. This is due to the 
minuteness of the particles of humus by which the aggregate surface for hold- 
ing water is materially augmented and partly, perhaps, to a direct power of 
imbibing water. The total effect is to decrease loss by percolation and evap- 
oration, and at the same time to raise the amount of non-available water. In 
more compact soils it increases the absorption of run-off, and possibly breaks 
up excessive loss by evaporation in consequence of capillarity. In the stiffest 
soils it also reduces the echard, correspondingly increasing the amount of 
water available to the plant. Humus is also associated with other reactions 
which affect the holard, such as weathering, preventing erosion, and protect- 
ing against evaporation. 

(11) Reaction by decreasing water-content. — Plants decrease the holard 
directly only by absorption and transpiration. This is a universal reaction 
of plant communities, and is often critical in the case of the seedlings of woody 
plants. It is characteristic of the ecotone between grassland and forest, and 
plays an important part in the persistence of the grassland subclimaxes, as 
in the prairies and plains. It doubtless has a similar effect on the seres of a 
forest region, but its influence is much less marked. The holard is also 
diminished as a result of other reactions. This is most striking in the case of 
the shallowing of the water by plant remains and by the deposition of silt in 
consequence of the obstruction by vegetation. 

NUTRIENTS AND SOLUTES. 

The reactions of plants which affect the soil solution are least understood, 
and hence most debated. The actual existence of some of them is still in 
controversy, and in but one or two cases has a definite relation to succession 
been demonstrated. The possible reactions upon the content of the holard are 
as follows: (1) by adding nutrients or actual food, (2) by decreasing nutri- 
ents, (3) by producing acids, and (4) by producing toxins. 

(12) Reaction by adding nutrients or foodshiffs. — This reaction is the 
direct consequence of the annual fall of leaves and the death and decomposi- 
tion of plants or plant parts. In this way a large supply of mineral salts is 
returned to the soil, and sooner or later these are freed to enter the soil solu- 
tion. It seems clear that this process favors plants with a high nutrient re- 
quirement, but this may be negligible where there is an abundance of nutrients 
in the soil. The whole question really hinges upon the relation between the 
amount returned each year and the amount already available in the soil. At 
any rate, we have no convincing evidence that humus plays an efficient role in 
succession apart from its fundamental relation to water-content. Experi- 
ment only can decide this matter, since nutrients and water are absorbed 
together and both would necessarily tend in the same direction. Cowles 
(1911:176) has suggested that glucose and other soluble food in the humus 
may be absorbed by green plants, but as yet there is no direct evidence of 
such utilization. 

(13) Reaction by decreasing nutrients. — The inevitable effect of the absorp- 
tion and use of solutes by growing plants is to decrease the total supply. 
Actually, however, this reduction is insignificant in nature, and probably also 
in cultivation. The amount absorbed each year is a very small part of the 



90 REACTIONS 

total amount present; so much so that even cultivation may effect no appre- 
ciable reduction in 50 years, as shown by the experiments at Rothamsted 
(Hall, 1905:36). In addition, all the nutrients absorbed are returned sooner 
or later, and in most communities the annual return must nearly counter- 
balance the use. In any event, there is no indication at present that success- 
sional movement is affected by the direct decrease of nutrients through ab- 
sorption. 

The formation of heath-sand or "bleisand" probably furnishes an example 
of reduction in nutrient-content as a consequence of another plant reaction. 
This is the formation of acids by humus. These render the mineral nutrients 
soluble, and the latter are then removed by the percolating water, beginning 
at the top. In extreme cases, little remains but quartz sand, which acquires 
a characteristic leaden color in consequence of the precipitation of particles 
of humus. Such "bleisand" represents in consequence the extreme of poverty 
in regard to soil nutrients. It makes the ecesis of more exacting species almost 
impossible, and thus secures the persistence of the heath stage for very long 
periods, so that it may often be regarded as a climax. 

(14) Reaction by producing acids. — The direct reaction of plants in excret- 
ing carbon dioxid from the root surface has already been considered under 
"Weathering." It is probable that this bears no relation to the production 
of acids in the more or less partial decomposition of humus. Wherever plant 
remains accumulate abundantly in water or moist places, access of oxygen is 
difficult. The decomposition is slow and partial, and the water or soil becomes 
more or less acid. The acids formed are very little understood, and the process 
by which they are formed is likewise obscure. Lack of oxygen seems a neces- 
sary condition of their production, and the effect of the acid upon plant 
growth is complicated with the effect of deficient aeration. Both appear to 
act together in diminishing the absorptive power of roots, probably in conse- 
quence of decreased respiration. This apparently places a premium upon 
plants with modifications for reducing transpiration, and acid areas are 
usually characterized by so-called "bog xerophytes" such as Ledum, Kalmia, 
Vaccinium, etc. In spite of much recent study, the nature of bog plants is 
still an open question. It seems increasingly evident that most of the xeroid 
species of wet places are not xerophytic at all, but that a restricted group 
characteristic of peat-bogs, heath-moors, etc., are actual xerophytes. Even 
with these, however, no final solution is possible until their water requirements 
have been studied experimentally and their transpiration response is known. 
In so far as succession is concerned, the production of acid in swamps may 
modify the normal reaction of decreasing water-content, and mark a series of 
stages which dominate for a time, owing to a favorable response to poor aera- 
tion. Whenever the latter is improved by drainage, filling, or a drier climate, 
conditions become more favorable to species of neutral or alkaline soils, and 
the bog plants disappear in consequence or as the outcome of competition. The 
work of Gates (1914) confirms the assumption that the bog heaths are the 
result of winter xerophily, while a recent study of the transpiration and 
growth of plants in aerated bog-water indicates that the acid is a concomitant 
only, and not a cause. 

(15) Reaction by producing toxins. — The question of the direct production 
of toxic substances by excreting plant roots is a much mooted question. 



SOIL ORGANISMS 91 

Without attempting to pass upon the matter in general, it may be said that 
the most persistent search for a decade has failed to reveal any evidence of 
their role in the innumerable examples of succession in the Rocky Mountains. 
On the contrary, the detailed study of the ecesis of occupants and invaders 
in the families and colonies of pioneer stages indicates better development 
in such areas, as would be expected from their reactions. 

The existence of bog toxins resulting from partial decomposition or from 
the complex organic interactions of bogs is much more probable (Livingston, 
1905; Transeau, 1906; Dachnowski, 1912). It is difficult to regard their pres- 
ence as proved, however, and a long period of quantitative and experimental 
study of succession is needed to reveal their importance as a reaction. At 
the present it seems clear that acids, poor aeration, and bog toxins would all 
have the same effect upon successional movement. The chief task before us 
is to assign to each one its proper place (for later views, cf. Clements, 1921). 

SOIL ORGANISMS. 

The relation of plants to the organisms in the soil is so complex that it is 
impossible to recognize all of the effects, or to distinguish the causes of many 
of them. For the present purpose it will suffice perhaps to draw a distinction 
between the organisms directly connected with the plant and those not in 
organic relation to it. The former may be included in the general term of 
parasites, though many are symbiotic, of course, while the latter are sapro- 
phytes. Animals as well as plants are found in both groups. The parasites 
may be regarded as a direct reaction of the plants, while the saprophytes are 
an indirect reaction, or, better, a consequence of the accumulation of plant 
remains. 

(16) Reaction by means of parasites. — The relation between host-plant and 
parasite is so intimate that it seems hardly to constitute a reaction. Yet 
it has a direct bearing upon the fate of the community and its part in succes- 
sion. The latter is determined largely by the degree of parasitism. If it is 
intense and destructive, the individual will be destroyed or handicapped in 
its competition or dominance. As a consequence, it may disappear wholly 
from the community, though this is relatively rare. The most usual effect is 
a decrease in number or dominance by which the species assumes a less 
important role. In the majority of cases no direct influence is discoverable, 
the effect being merged in the general outcome of competition. 

When the relation is more or less symbiotic, its general effect is first to 
increase the dominance of the host-plant, but finally to favor species with 
higher nitrogen demands. Warren (1909) has pointed out that this is the 
effect of the nodule-bearing legumes in the prairie formation. The legumes 
are able to grow in the poorer soils by virtue of their symbiotic partnership 
and consequent nitrogen production. They thus make possible the greater 
development of grasses, before which they disappear, sometimes completely. 
The presence of mycorrhiza alone makes possible the successful ecesis of an 
increasing number of plants, especially trees and shrubs, and hence controls 
their appearance in succession. Their disappearance may be due to the 
competition resulting from the invasion of plants with greater nitrogen 
demands, but it is also influenced by other reactions. 



92 REACTIONS 

(17) Reaction by means of saprophytes. — These have to do chiefly with the 
formation of humus or with its modification in such a way as to make its 
nitrogen again available for plants. This is true even of those fungi which 
exist in the soil as saprophytes, and become parasitic when the proper host 
becomes available. A few of these are very destructive in their action, and 
sometimes effect the complete disappearance of a dominant. The fleshy fungi 
which play a large part in the ground layer of boreal and mountain forests 
have to do largely with hastening the conversion of plant remains into humus^ 
with its attendant effects upon water-content, nutrients, etc. This is the 
well-known role of a large number of soil bacteria, especially those which 
free ammonia or elaborate nitrates from nitrogenous substances or fix free 
nitrogen. In the case of both fleshy fungi and bacteria, the final effect is to 
produce conditions in which plants with greater requirements can enter and 
displace those with less exacting demands. The same general effect is exerted 
by animals living in the soil, though there is some evidence that protozoa may 
play an antagonistic role. 

AIR REACTIONS. 

The reactions of plant communities upon atmospheric factors are less 
numerous and usually less controlling than those upon soil. The notable 
exception is the reaction upon light, which plays a decisive part in the later 
stages of the majority of seres. The effects upon the other air factors are so 
interwoven that it seems best to consider the reactions upon humidity, tem- 
perature, and wind together. As a consequence, the reactions may be grouped 
as follows: (1) upon light; (2) upon humidity, temperature, and wind; (3) 
upon the local climate; (4) upon aerial organisms. 

(18) Reaction upon light. — The primary reaction upon light is seen in the 
interception of sunlight and the production of shade of varying degrees of 
intensity. There may also be a secondary effect upon the quality of the light 
(Zederbauer, 1907; Knuchel, 1914) where it has to pass through a dense 
canopy of leaves. The preponderance of results up to the present time indi- 
cates that the light beneath the tree-layer passes between the leaves and not 
through them, and is essentially unchanged as to quality. The reduction of 
light intensity is usually slight or even lacking in the early stages of succes- 
sion, though exceptions occur whenever plants are tall and dense, as in con- 
socies of Phragmites, Spartina, or Typha, or when leaves are broad and 
spreading, Nymphaea, etc. As the population becomes denser, it intercepts 
more and more light, with the result that a subordinate layer appears. With 
the entrance of shrubs and trees, the reaction steadily becomes more marked 
and the demarcation of subordinate layers more striking. In a layered 
forest the reduction in light value is a progressive one from the primary 
layer downward. In many forests of this type the cumulative reaction is so 
complete that the ground layer can consist only of fungi and mosses, the 
latter with the lowest of light requirements. As the canopy becomes denser 
and denser, either by the growth of individuals or by the entrance of trees 
with closer tops, the layers begin to disappear. This usually takes place in a 
downward direction, the final stage of a closed forest containing only mosses, 
fungi, and saprophytic phanerogams, with occasional low herbs. Thus, even 



AIR REACTIONS 93 

after the establishment of a dominant species of a climax stage, there may 
still be a successional disappearance of the subordinate layers. 

The most important effect of the reaction upon light is shown in the suc- 
cession of dominants after one or more have secured the controlling position 
with respect to light. This is seen most clearly and is best understood in 
the case of trees, but it is true of shrubs and in some degree of grasses and 
herbs. To maintain itself, a species of forest tree is confronted by the two- 
fold task of being able to grow in both sun and shade. If it is the first tree 
to invade, the crucial test comes when it has reacted upon the light in such 
a way as to make it necessary for its seedlings to ecize in the shade. This is 
a test in which practically all forest pioneers fail. The species which invade 
the pioneer forest must grow in reduced light intensity for a long time, until 
the individuals stretch above the original trees. The change of the leafy top 
from shade to sun is an advantage, however, and it marks the beginning of 
the disappearance of the trees of the first forest stage. The reaction of closer 
growth, denser crowns, or both, decreases the light still further, with the 
result that the seedlings now meet a severer test than did those of the preced- 
ing generation of the same species. In most cases they are able to establish 
themselves, but in smaller number and with reduced vigor. They are placed 
at a disadvantage in competing with the seedlings of species that endure 
deeper shade. When these enter they soon gain the upper hand, reach up 
into the dominant layer, and gradually replace the species already in occu- 
pation. In most, if not all regions with a forest climax, this pnocess may be 
repeated several times, until the species whose seedlings endure the lowest 
light intensity are in final possession. 

This succession of tree dominants was probably first clearly perceived by 
Dureau de la Malle (1825), but the explanation of its relation to light was 
first suggested by Vaupell (1857). It was long known to foresters as the 
' ' alternation of essences, ' ' and the essential response to reduced light intensity 
has been termed ' ' tolerance. ' ' A table of tolerance which arranges the species 
of trees of the same climatic region in the order of decreasing light require- 
ment gives also their successional relation. The earliest tolerance table was 
probably that of Vaupell. The first experimental determination by shading 
seedlings was that of Kraft (1878), which gave the following order: (1) 
Pinus, (2) Betula, (3) Fraxinus, (4) Picea, (5) Acer, (6) Carpinus, 
(7) Fagus and Abies. This table was not based upon the study of succession 
as was that of Vaupell. In the last decade or two various tables have been 
proposed on different bases for the native and exotic forest trees of Europe. 
For American species, Zon and Graves (1911) give a fairly complete group- 
ing, but this does not permit a contrast of the associated species of a climax 
area. The most fundamental test of tolerance is perhaps the actual sequence 
in succession under natural conditions, supplemented by photometric deter- 
minations of light intensity in various situations. This method has given the 
following order for the central Rocky Mountains: (1) Pinus murrayana; 
(2) Popuhts tremuloides; (3) Pinus pondcrosa, P. flexilis: (4) Pseudotsuga 
mitcronata; (5) Picea engelmanni; (6) Abies Imiocarpa (Clements 1910). 

Fricke (1904) has shown by experiment that competition for water enters 
into the consideration of tolerance. By cutting trenches around isolated 
groups of seedlings of Pinus silvcstris, he destroyed the root competition of 



94 REACTIONS 

the parent trees without changing the light values. In the first summer the 
growth of the seedlings within the area much exceeded that of those outside, 
while a totally new and vigorous herbaceous layer developed. He also deter- 
mined the holard of soils with and without living roots, and found the latter 
to contain 2 to 6 times as much water. This emphasizes the influence of 
water-content in the later stages of succession and the degree to which com- 
petition can modify it. It also makes it plain that the more obvious effects 
of light in these same stages must be checked by the quantitative study of 
the water relations. 

(19) Reaction upon, humidity, temperature and wind. — These three factors 
are necessarily linked together because of their direct effect upon the plant 
through transpiration and the indirect effect through the evaporation of soil- 
moisture. The plant community reacts directly upon each factor, and these 
act upon each other, but the response of the plant is controlled by humidity. 
The reaction of a sparse pioneer population is more or less negligible, but the 
increasing density and height of the individuals bring about a measurable 
result, which becomes significant in most closed associations, especially those 
of shrubs and trees. In layered forests the reaction is greatest in the ground 
layer or beneath it, where it consists of herbs. Humidity is directly increased 
by transpiration, but the effect is cumulative because the moisture-laden air 
is not carried away. The heat rays are absorbed or reflected, and the lower 
temperature that results causes an increase in relative humidity. The capac- 
ity of the air* for moisture is correspondingly decreased and both transpira- 
tion from the plants and evaporation from the soil-surface are reduced. The 
final effect is to make the water-content more efficient and thus essentially to 
increase it. The general effect of the reaction is the same as that of increasing 
humus, and the two are indistinguishable as a rule. The reduced evaporation 
from the surface soil, and perhaps from the seedlings as well, is a critical 
factor in the ecesis of many seedlings, especially those of trees. 

(20) Reaction upon local climate. — Plant communities react upon the air 
above them by transpiration and by lowering the temperature. As a conse- 
quence, they receive more soil-moisture as dew and rain than do bare areas. 
This reaction of vegetation is measurable only in the case of forest and scrub, 
but probably occurs in some degree in all vegetation, particularly in the 
formation of dew. The effect of wooded areas upon rainfall has long been 
a subject of controversy, but the evidence in favor of a positive reaction is 
now available from so many sources that it seems conclusive. Zon (1912:205) 
has made the most recent summary of the evidence that forests increase rain- 
fall. At Nancy the average increase in forested areas for 33 years was 23 
per cent, while in Germany and India it was computed to be 12 per cent. 
A four years' experiment to check out the possible error due to faulty instru- 
ments yielded an excess of 6 per cent for the forest. Observations in the 
north of Germany indicate that the influence of forest increases rapidly with 
the altitude. At elevations less than 300 feet the effect was negligible, while 
at altitudes of 2,000 to 3,000 feet it ranged from 19 per cent to 84 per cent. 
Denuded mountains often fail to cause moisture-laden winds to precipitate 
their moisture, as Angot has shown to be the case in Spain. A similar influ- 
ence is often exerted by the hot, dry gravel ridges about Pike's Peak upon 
the local showers in mid-summer. 



AIR REACTIONS 95 

Weber found the annual rainfall near Nancy to be 4 inches greater at a 
forest station than in one situated in a denuded area. Observations by 
Miittrich of the effect of forestation upon the rainfall of the Liineberg heath 
showed that the precipitation increased steadily during a 7 years' period, and 
finally exceeded that of adjoining areas. Similar results were obtained after 
a plantation had been made in the steppes of southern Russia, where the 
average rainfall from 1893 to 1897 was 17.9 inches in the steppe and 22.2 
inches in the newly established forest. Blandford found that the new forest 
growth in a protected area in British India had a decisive effect upon the 
rainfall, increasing it from 2 to 12 inches at various stations. Fautrat has 
made observations which not only show that the rainfall above tree-tops is 
greater than in the open, but also that it is appreciably greater above conif- 
erous than above broad-leaved forests. These were confirmed by the rainfall 
recorded under broad-leaved and coniferous canopies. In 1876 the soil under 
the former received 16.7 inches and that under the latter only 11 inches. 

Ney determined the amount of dew and frost condensed by leaves in north- 
ern latitudes to be as much as 0.4 to 0.8 inch per year. On the Pacific coast 
of North America and in tropical regions the condensation must be very much 
greater. There are no conclusive observations as to the height at which the 
cooling effect of a forest is felt, but Zon (219) cites the statement of Renard 
that this has repeatedly been noticed at an elevation of 5,000 feet during 
balloon ascensions. 

R. von Hohnel, in the study of oak forests in Austria from 1878 to 1880, 
found that an acre of oak forest 115 years old absorbed from 2,200 to 2,600 
gallons of water per day. This corresponds to a rainfall of 3 to 4 inches per 
month, or a rainfall of 17.7 inches for a vegetation period of 5 months. Zon 
cites also the experiments of Otozky to the effect that forest, on account of its 
excessive transpiration, loses more water than grassland or a bare area. He 
concludes that the transpiration of forests has a critical effect upon the rain- 
fall of continents, since the amount of water consumed by a forest is nearly 
equal to the total annual precipitation. Bruckner concluded that the vapor 
evaporated from the peripheral areas of continents, i. e., the 79 per cent of 
land surface which drains directly towards the ocean, is able to supply seven- 
ninths of the precipitation over such areas. From the balance-sheet of water 
circulation over the earth's surface, Bruckner reached the conclusion that 20 
per cent of the vapor comes from evaporation on land, that only 7 per cent of 
the evaporation from the ocean reaches the land as rainfall, and that 78 per 
cent of all the precipitation over the peripheral land area is furnished by this 
area itself. While his conclusions are in accord with the facts so far as 
known, it is evident that their acceptance is impossible without much more 
exact study of evaporation and transpiration, as well as of the rainfall of 
many regions. 

(21) Reaction upon aerial organisms. — As in the case of soil organisms, 
this may be the direct consequence of the presence of the host-plant or matrix, 
or it may be the indirect result of the reaction upon the air factors. As a rule, 
the two effects are correlated, the presence or the success of the parasitic or 
saprophytic organism being affected by the conditions as well as controlled 
by the host-plant or matrix. This reaction is characteristic of communities 
with a dominant canopy, such as forest and thicket, but obtains in some 



96 REACTIONS 

degree in all vegetation. It is most obvious in the development of lichen 
families and colonies, and has an interesting and probably important relation 
to the presence and behavior of pollinating insects. 

Correlation of reactions. — The efficient reactions in the great majority of 
seres are those that have to do with the increase or decrease of water-content 
and the decrease of light intensity. These are the controlling reactions in 
all primary seres, though a portion of the development may be dominated by 
the air-content or by toxins, as in peat-bogs, or by the nutrient relations, as 
in heath. Up to the appearance of the first shrub stage the water-content 
reactions are directive. With the entrance of trees and shrubs it becomes 
chiefly or largely subordinate to the light reactions. In the development of 
grass or herbaceous climax formations, reaction upon light plays a smaller 
part. On the contrary, many secondary seres, especially those originating 
in bums or clearings, may be controlled almost entirely by the decreasing 
light value. In short, the chief reaction upon the habitat is necessarily upon 
the soil and its factors, until the community develops sufficient height and 
dominance to control air conditions. 

The accumulation of plant remains or humus is the most complex of &]} 
reactions, as it is the most universal, since it is the direct and inevitable out- 
come of the presence of plants. In initial water or wet areas it decreases 
water-content and increases nutrient-content and aeration, unless decomposi- 
tion produces an excess of acids or other deleterious substances. Its effects 
in dry areas are largely opposite. It is the great factor in increasing the 
water-content, but at the same time it also increases the available nutrients. 
With the appearance of woody communities its influence is masked by the 
light reactions, but continues to be felt in some degree. It becomes obvious 
again in woodlands where conditions cause the development of acids, as in 
beech peat, and may lead to a critical decrease in water-content. 

Quantitative study of reactions. — Our exact knowledge of the amount and 
effect of community reactions is very slight. The investigation of habitat 
and community by means of instruments is still exceptional. The few quan- 
titative studies so far made have been directed for the most part to other 
problems, and have rarely dealt with the measurement of reactions. The 
earliest attempts to measure the reactions of the stages in succession were 
made in the woodland and prairie formations of Nebraska from 1898 to 1906, 
and in the mountain and plains formations of Colorado from 1901 to 1910. 
As already indicated, the first account of the quantitative study of the major 
reactions of a succession was published in 1910, in connection with the life- 
history of the secondary sere in burned areas. This was followed by a similar 
account of the reactions in the grassland stages of the Great Plains (Shantz, 
1911). The earlier results in Nebraska and Colorado have as yet been pub- 
lished only in part (Thornber, 1901; Hedgcock, 1902; Clements, 1904; E. S. 
Clements, 1905; Shantz, 1906). 

In addition to the pioneer work of Wiesner (1895, 1904, 1907) upon the 
reaction on light, a number of measurements have been made during the last 
decade of habitat factors. While these were not directed at reactions as such, 
they are often of much value in this connection. Such are the studies of 
Livingstone (1906) on the relation of desert plants to holard and evaporation, 
Zederbauer (1907) on the composition of forest light, Yapp (1909) on evap- 



QUANTITATIVE STUDY OF REACTIONS 97 

oration and temperature in swamps, Dickey (1909), Brown (1910), and 
Sherff (1913) on evaporation, Knuchel (1914) on quality of forest light, 
and of a number of others who have investigated bog reactions or evaporation. 
The first special study of evaporation and succession was made by Transeaii 
(1908) in the study of Long Island vegetation. Dachnowski (1912) has 
studied the reactions in bog habitats, and Fuller (1911, 1912, 1913, 1914) 
has investigated the relation of water-content and evaporation to the develop- 
ment of the cottonwood-dune association and the oak-hickory association. 
Gleason and Gates (1912) have made similar studies of evaporation in various 
communities in central Illinois. Pool (1914) has recently investigated the 
water relations of sandhill seres, and Weaver (1914) has studied the relation 
of evaporation to succession in the Palouse region of Idaho and Washington. 



VI. STABILIZATION AND CLIMAX. 

Stabilization. — The progressive invasion typical of succession everywhere 
produces stabilization. The latter is the outcome of greater occupation due 
to aggregation and migration, and of the resulting control of the habitat by 
the population. In other words, stabilization is increase of dominance, cul- 
minating in a stable climax. It is the mutual and progressive interaction of 
habitat and community, by which extreme conditions yield to a climatic 
optimum, and life-forms with the least requirements are replaced by those 
which make the greatest demands, at least in the aggregate. So universal 
and characteristic is stabilization that it might well be regarded as a synonym 
of succession. It has the advantage of suggesting the final adult stage of the 
development, while succession emphasizes the more striking movement of 
the stages themselves. 

Causes of stabilization. — The essential cause of stabilization is dominance. 
The latter is partly due to the increasing occupation of a bare area, but is 
chiefly the result of the life-form. The occupation of annuals in an initial or 
early stage of a secondary sere is often complete, but the dominance is usually 
transient. Effective dominance can occur only when the prevailing life-form 
exerts a significant reaction, which holds the population in a certain stage 
until the reaction becomes distinctly unfavorable to it, or until the invasion 
in force of a superior life-form. Dominance is then the ability of the charac- 
teristic life-form to produce a reaction sufficient to control the community 
for a period. Dominance may mean the control of soil factors alone, pri- 
marily water-content, of air factors, especially light, or of both water and 
light. Initial life-forms such as alga?, lichens, and mosses are characteristic 
but not dominant, since the reaction they produce prevents control rather 
than gives it. This is the essential difference between the initial and the 
final stages of succession. While both react upon the habitat, the reaction of 
the one favors invaders, that of the other precludes them. The reactions of 
the intermediate stages tend to show both effects. At first the reaction is slight 
and favors the aggregation of occupants; then it becomes more marked and 
produces conditions more and more favorable to invasion. On the other hand, 
when the reaction is distinctly unfavorable to the occupants, the next stage 
develops with greater rapidity. Each stage is itself a minor process of 
stabilization, a miniature of the increasing stabilization of the sere itself. 
Reaction is thus the cause of dominance, as of the loss of dominance. It 
makes clear the reason why one community develops and dominates for a 
time, only to be replaced by another, and why a stage able to maintain itself 
as a climax or subclimax finally appears. Thus, reaction furnishes the expla- 
nation of stabilization, as it does of the successive invasions inherent in suc- 
cession. 

Relation to the climax. — The end of the process of stabilization is a climax. 
Each stage of succession plays some part in reducing the extreme condition in 
which the sere began. It reacts to produce increasingly better growing condi- 
tions, or at least conditions favorable to the growth of a wider range of 
species. This is equivalent to reducing an excess of water-content or remedy- 



DEGREE OF STABILIZATION 99 

ing a lack of it. The consequence is that the effect of stabilization on the 
habitat is to bring it constantly nearer medium or mesophytic conditions. 
Exceptions to this occur chiefly in desert regions, though they may occur also 
in water areas, where processes of deposit and erosion alternate. The effect 
upon the plant population is corresponding. The vast majority of species are 
not pioneers, i. e., xerophytes and hydrophytes, but mesophytes with com- 
paratively high but balanced requirements for ecesis. For this reason the 
number of species and individuals grows larger in each succeeding stage, 
until the final dominance of light, for example, becomes restrictive. At the 
same time the life-forms change from those such as lichens or submerged 
plants with a minimum of aggregate requirements to forms with an increas- 
ingly high balanced need. The period of individual development increases as 
annuals are succeeded by perennials and the latter yield to dominant shrubs 
and trees. The final outcome in every sere is the culmination in a population 
most completely fitted to the mesophytic conditions. Such a climax is perma- 
nent because of its entire harmony with a stable habitat. It will persist just 
as long as the climate remains unchanged, always providing that migration 
does not bring in a new dominant from another region. 

Degree of stabilization. — Apart from the temporary stability of each suc- 
cessional stage, the final stabilization of a sere varies greatly in permanence. 
In the actual seres of the present time this is best illustrated by the water sere 
in a region where moor and heath appear as stages on the way toward the 
forest climax. As a consequence of peculiar soil reactions each one is usually 
a subclimax of unusual duration, and under the artificial conditions evoked 
by man may persist as an actual climax. A similar effect occurs locally in 
the Rocky Mountains, where springs keep the soil too moist for the pines 
which normally succeed aspens on dry slopes. The result is that the aspen 
remains dominant through a period equal to several stages, and yields only 
when the final spruce and fir become controlling. This persistence of the 
aspen is doubtless promoted by repeated fire, which is a universal cause of 
apparent stability. This is certainly a large factor in the subclimax prairie. 
Whatever the origin of prairie may have been, its extent and duration are 
partly due to the effect of fire upon woody communities, followed by a similar 
influence produced by clearing and cultivation. In all cases of subclimaxes, 
i. e., of premature stabilization, the activities of man will nearly always prove 
to be concerned in a large degree. 

In the analysis of existing seres it seems evident that complete stabilization 
occurs only when the climax is controlled by trees, which are the most 
dominant and hence the highest ecologically of all the life-forms. Develop- 
mentally, all other final communities are subclimaxes of greater or less dura- 
tion; actually, they may exist throughout one or more successional periods. 
They may owe their existence to any of the following factors: (1) climatic 
control; (2) reaction upon the soil; (3) interference by man; (4) exclusion 
by barriers constituted by later dominants. The removal of the check permits 
complete development and the appearance of the serai climax. The evolution 
of a new vegetation through long periods of time produces new climax forma- 
tions and leads to corresponding seres. In the complex successional develop- 
ment of vegetation, since the first appearance of land areas, all possible 
degrees of stabilization have occurred, with the exception of complete develop- 



100 STABILIZATION AND CLIMAX' 

mental stability. The latter can never occur in vegetation as a whole as long 
as plants are evolved or conditions changed. Fortunately, our real concern 
with stabilization is limited to the degree in which it appears in each sere. In 
other words, it requires study as a developmental phenomenon, and not as 
a more or less active condition. 

LIFE-HISTORY STAGES. 

Nature. — While the movement from initial stage to climax or subclimax is 
practically continuous, there are typically certain periods of comparative or 
apparent stabilization. These correspond to population or invasion maxima, 
which mark more or less well-defined stages or communities. As noted else- 
where, such stages usually appear much more distinct than they really are, 
owing to the fact that the study of succession so far has been little more than 
the arrangement in probable sequence of stages contemporaneous in different 
areas. However faint their limits, real stages do exist as a consequence of 
the fact that each dominant or group of dominants holds its place and gives 
character to the habitat and community, until effectively replaced by the next 
dominant. The demarcation of the stages is sharper when the change of 
population is accompanied by a change of life-form, as from grassland to 
scrub or forest. In some secondary seres there is little or no change of life- 
form and the stages are few and indistinct. In rare cases the dominants of 
the entire sere may be present the first year after a burn, for example, and 
the well-marked stages are due solely to the rate of growth, which causes the 
dominants to appear and characterize the area in sequence. 

Kinds of stages.- — Stages may be distinguished upon various bases. The 
most obvious distinction is based upon change of population. This is the 
readiest method, but also the least significant, unless it takes account of 
dominance as well. Change of life-form is more fundamental and equally 
convenient, while change of the habitat is even more significant, though much 
harder to recognize. Dominance, with reaction, includes all of these bases, 
and is by far the best method. The essential stages are those marked by a 
dominant or group of dominants. For complete analysis, however, it is 
desirable to recognize other stages, such as those based upon population and 
upon effective change of habitat. For general purposes, also, it is convenient 
to distinguish stages with reference to their position in the course of develop- 
ment. As a consequence, the best method of treatment is to base stages upon 
successive dominants and to recognize substages whenever a change of char- 
acter makes it desirable or necessary. This is usually in the early part of 
seres, before dominance is clearly established. At the same time it is helpful 
to group stages for reference or to bring out certain relations. They may be 
grouped into initial, medial, or final, or into temporary or migratory, on the 
one hand, and permanent, stable, ultimate, or climax on the other. As to 
habitat, one primary sere, for example, may show rock, gravel, grassland, 
and woodland stages, and another water, sedgeland, grassland, and woodland. 
The corresponding life-form stages would be lichen, moss, herb, grass, scrub, 
forest, and algae, herb, sedge, grass, scrub, forest. 

Role of life-forms. — Since dominance and reaction are consequences of 
the life-form, it follows that the main stages in development are marked by 



LIFE-HISTORY STAGES 101 

different life- forms. The latter is used in a broader sense than is usual; it 
includes not only the vegetation form, with its synonyms, biological forms, 
growth-forms, etc., but also the habitat forms, and something of the repro- 
duction form as well. The life-form, in short, comprises all of the structures 
which mark the species as an ecological agent. Its fundamental correspond- 
ence with the habitat is obvious. The forms of the aerial shoot are of the 
first importance, but the organs of perennation have to do directly with 
occupation and with ecesis. The root- forms are usually of secondary im- 
portance, though in sand and gravel in particular they play a conspicuous 
role. In essence, the life-form is the superposition of water and light adapta- 
tions upon the vegetation form, though in cryptogams especially, the latter 
corresponds closely to the reproduction or taxonomic form. 

It is difficult to refrain from speaking of life-forms as lower and higher 
with respect to their position in succession. This is determined by their 
demands upon the habitat as well as by their reaction. In the case of the 
pioneers of most primary seres this is warranted by the taxonomic develop- 
ment as well, and there can be little objection to this as a convenient compari- 
son. Because of their universal presence, the plankton alga? of water-bodies 
are hardly to be regarded as pioneers in a particular water sere, though this 
is their position in the geosere. The actual pioneers of a water sere are 
charads, submerged mosses and flowering plants, with a life-form character- 
istic of the habitat. Probably submerged attached algae belong here also. 
Floating forms, primarily phanerogams, mark the first division of the habitat 
into two media, water and air, and serve as a natural transition to the reed 
form. In this there is a complete differentiation by the two media into aerial 
shoot and aquatic roots and shoot. In many cases it is desirable to distin- 
guish the sedge form from the reed, though there is manifestly no sharp line 
between them. This is true of the grass form in some measure, but it is 
clear that the habitat has changed materially as a rule. The change from 
grassland to woodland is the most significant, since the persistence of the 
stems greatly emphasizes the reaction upon light and other air factors. While 
the woody form is consequently sharply distinguished, this is not always true 
of the subordinate forms, bushes, shrubs, and trees, since the difference is 
primarily one of size. In spite of its aerial position, Sphagnum is essentially 
a submerged moss. In many cases it is clearly a pioneer life-form, though its 
ability to bring about the swamping of vegetation complicates its treatment. 
The shrubs characteristic of heath belong to a peculiar habitat modification 
of the shrub form, produced directly or indirectly by acid soil, by deficient 
aeration or by winter. 

In rock seres, the pioneer life-form is the alga, when the rocks are wet, and 
the lichen when they are dry. It is interesting, if not significant in this con- 
nection, that the alga is an essential part of the lichen pioneer. In fact, it 
seems probable that alga?, especially Pleurococcus, may become established on 
exposed rocks during wet periods and thus actually precede the lichens. 
Such must be the case with rock lichens in which the spores are still efficient. 
On moist rocks, alga? may also be followed by lichens, especially Collemaceae. 
though the algal character of moss protonema enables the mosses to appear 
quickly, and often, it would seem, must permit them to be the first pioneers. 
On dry rocks there is a fairly distinct successional difference between the 



102 STABILIZATION AND CLIMAX 

crustose and foliose lichen forms. The moss form, with its minute rhizoids 
and power of withstanding desiccation, quickly follows the lichen stages and 
may even precede the foliose lichens. The pioneer herb form on exposed 
rock has the mat habit as a rule and resembles the moss cushion in many 
respects. It is quite different in character from the forms of herb and grass 
which grow in the rock clefts. These belong essentially to the next stage, as 
they actually grow in soil and are only apparent rock plants. With the 
appearance of grasses and herbs, the later life-forms of the rock sere become 
the same as in the water sere. 

The sequence of life-forms in secondary seres is essentially the same as in 
primary ones. A characteristic exception, however, is furnished by the fact 
that the pioneer life-forms are perhaps never the same. The approach is 
sometimes very close, as, for example, when mosses appear after a burn. 
In practically all such cases flowering plants develop the same year, and the 
mosses, as well as possible algas and lichens, never form a characteristic stage 
which persists for several or many years. In fact, the very nature of 
secondary succession as a course of development less complete than the 
primary one precludes its beginning with the original initial stage. 

Reasons why plants disappear. — Stages are obviously the result of the dis- 
appearance of occupants and the appearance of invaders. The causes of the 
disappearance of plants are thus in large part the explanation of the stages 
themselves. Most species disappear wholly, though some persist through 
more than one stage, usually in this case becoming subordinate. Others are 
reduced to a small or insignificant number of individuals, which may persist 
as relicts for a long time. Plants disappear for one or more of the following 
reasons: (1) unfavorable conditions due to reaction; (2) competition; (3) 
unfavorable conditions or actual destruction due to parasites, animals, or 
man; (4) old age. The first two are the universal causes of disappearance, 
and while reaction is much the most important, its effect is distinguished 
with difficulty from that of competition. Complete, or nearly complete, 
destruction of a community results in secondary succession. It is only 
when the destruction operates upon the dominant or dominants alone that 
a change of stage may occur without clearly producing a secondary sere. 
This may occur in the selective lumbering of a mixed forest, and in grazing 
when not too close, but there is a question in both cases whether this is nof 
really imperfect secondary succession. The influence of old age in the dis- 
appearance of dominants is far from evident. It seems important in deciding 
the competition between short-lived trees, such as aspens and birch, and long- 
lived conifers, and in the resulting dominance of the latter. But it is quite 
possible that this is really due to differences in growth and especially in 
height. In the case of pioneers with radial growth, such as lichens, cushion 
herbs, and grass, the death of the central portions seems due to what may 
well be called old age. This process sometimes extends throughout the 
whole mat, and is apparently a factor of some importance in the disap- 
pearance of the mat pioneers of alpine gravel-slides, as in that of rock 
lichens. 

Reasons why plants appear at certain stages. — Migrules are carried into 
an area more or less continually during the course of its development. This 
is doubtless true of permobile seeds, such as those of the aspen. As a rule, 



LIFE-HISTORY STAGES 103 

however, species reach the area concerned at different times, the time of 
appearance depending chiefly upon mobility and distance. As a consequence, 
migration determines in some degree when certain stages will appear. The 
real control, however, is exerted by the factors of the habitat, since these 
govern ecesis and hence the degree of occupation. The habitat determines 
the character of the initial stage by its selective action in the ecesis of the 
migrules. In all secondary areas, however, it must be recognized that the 
conditions of the habitat are largely due to the reactions of the original vege- 
tation. After the initial stage the development of succeeding ones is pre- 
dominantly, if not wholly, a matter of reaction, more or less affected by com- 
petition. In addition, some stages owe their presence to the fact that certain 
species develop more rapidly and become characteristic or dominant, while 
others which entered at the same time are growing slowly. This is a frequent 
explanation of stages of annuals, as also of stages of perennials preceding 
scrub or forest in secondary succession. 

Reasons why plants appear before their proper time. — The appearance of 
a species before its usual place in the sequence is generally due to migration 
in such amount that the handicap of more or less unfavorable conditions is 
overcome. It is most frequent in secondary seres, where the factors are less 
extreme, and the majority of the species can become dominant as soon as a 
sufficient number of migrules appear. In primary succession, especially, 
species can become characteristic only after the reactions have reached a cer- 
tain point. In the great majority of cases where a species appears out of 
order, it is due to local variations in the area. The premature development 
of an entire stage is caused by agencies which suddenly or rapidly change the 
habitat in the direction of the reaction. This is particularly true of areas which 
are affected in this way by animals or man. The number of stages omitted 
will depend upon the rate and degree of change. It is not unusual for this 
telescoping effect to eliminate two or more stages. The agencies which accel- 
erate reaction may also retard it, so that stages may be delayed by the undue 
persistence of an earlier one. In all secondary successions the time of appear- 
ance of shrub and tree stages depends in the first degree upon the action of 
the denuding agent. When this destroys all seeds and propagules, the se- 
quence of stages will be determined as usual by the mobility of migrules and by 
the habitat. When seeds or living parts of dominants escape destruction, the 
species concerned will take possession at once, or as soon as their development 
permits. Thus when an aspen forest is burned the root-sprouts often make 
the aspen again dominant the following season, and succession is found only 
in the renewal of the undergrowth. As noted in other connections, the seeds 
of lodgepole pine and similar pines are available in large numbers after fire, 
with the result that lodgepole pine reappears the first season, though its slow 
growth to dominance permits the rapid development of several stages. A 
similar effect has been noted by Hofmann in the forests of the Pacific slope 
when burned. The seeds of various species lie dormant for several years at 
such a depth in the forest duff or soil that they escape the fire and are ready 
for germination the year following. 

Initial stages. — No sharp line exists between initial and medial stages. 
The distinction, though convenient, can be only relative. Seres vary greatly 
in the number of stages and especially in the number and character of initial 



104 STABILIZATION AND CLIMAX 

stages. The number of stages may range from one to twenty or more, and a 
large number of secondary seres consist of not more than three or four. 
Furthermore, since secondary succession always begins after the pioneer stage 
of a primary sere, and usually at a medial or climax stage, the initial stages of 
the two are necessarily very different in character. Their one point in com- 
mon is the position at the beginning of the course of development. As a 
consequence, it is very convenient in analysis to use the term for the early 
stages of either kind of sere, but always with the fact that it refers to position 
and comparative characters clearly in mind. With more exact knowledge of 
succession, and of the relation of the various secondary seres to the primary 
or parental one, it will be possible to assign secondary initial stages to their 
proper developmental position. 

The initial stages of primary seres are marked by extreme physical condi- 
tions and by correspondingly specialized life-forms. Such primary areas as 
open water, rock, dune-sand, etc., occur throughout the world. The life-forms 
produced by them are likewise universal and, more interesting still, are highly 
mobile for the most part. Consequently, the pioneer aquatics of water areas, 
the lichens and mosses of rocks, the xerophytic grasses of dunes, and the halo- 
phytes of salt areas, consist of much the same species throughout the northern 
hemisphere, and some of them occur in tropical and austral regions. Hence 
the initial stages of water, rock, dune, or saline seres may be nearly or quite 
identical in widely separated regions, with the result that the seres concerned 
show increasing divergence to the various climaxes. From the extreme nature 
of primary areas, and of the plants in them, initial stages persist for a long 
time, largely because of the slowness of reaction and the incomplete occupa- 
tion. Primary areas differ much in these two respects. The greatest duration 
is found in the initial stages of a rock sere. The stages of a water sere follow 
each other more rapidly, and those of a dune still more rapidly, though the 
extent of the area in both cases plays a part (plate 12, a, b). 

The general limit of initial stages is indicated by a marked change in the 
extreme nature of the habitat and also by the degree of occupation in most 
cases. Both of these are more or less closely associated with the accumulation 
of humus. In water the initial stages are best regarded as three or four, 
ignoring the plankton. They are (1) the submerged stage, (2) the floating 
stage, (3) the reed stage, (4) the sedge stage. It is obvious that any one or 
more of these may be lacking, just as any one may be represented by a single 
consocies, or even more imperfectly. In all of them the occupation is fairly 
exclusive, and the reed and sedge communities are nearly or quite closed. The 
initial conditions on rock vary greatly, and the initial stages are correspond- 
ingly diverse. The longest series occurs on igneous rocks in dry or alpine 
regions. The number of stages to a more or less closed community on a soil 
with considerable humus is usually five: (1) crustose lichens, (2) foliose 
lichens, (3) mosses, (4) cushion plants, (5) herbs and grasses. When the 
rock disintegrates into sand or gravel the fourth stage often consists of bunch 
and mat plants. In dunes and other primary areas, fans, deltas, etc., the 
number of initial stages is often as few as one or two, though this depends 
much upon water relations and the adjoining vegetation. In all of these the 
earliest stages of the water or rock sere are excluded, because the soil forma- 
tion has already taken place. A deposit in water, for example, may begin its 



CLEMENTS 




A. Initial stages of a xerosere, lichens, mosses, and liverworts, Picture Rocks, 

Tucson, Arizona. 

B. Initial stage of a hydrosere, Xtiniphaca pot ysepala, in Two Ocean Lake. 

Yellowstone Park, 



THE CLIMAX 305 

development at the floating, the reed, or the sedge stage, just as rock may 
disintegrate without the presence of lichen or moss stages, and the succession 
begin with the development of herbs or grasses. 

Of all the initial stages, the first is in many ways the most significant. In 
consequence, it seems desirable to distinguish it as the pioneer stage. This 
term is most applicable to the extreme conditions of a primary area, though 
two kinds of pioneer stages may well be distinguished, as already suggested. 
Lichens, on the one hand, and submerged plants on the other, are the usual 
pioneers for rock and water seres respectively. For the present it seems best 
to designate only the first initial associes of the primary sere as the pioneer 
stage, and to leave the further distinction between actual and normal pioneer 
stages for future needs. The case of the first stage of secondary seres is 
different, however. The initial conditions are rarely extreme and the invasion 
is correspondingly extensive and rapid. The invaders do not meet pioneer 
conditions in the sense of primary areas, and the first stage is very short, 
often lasting but a year or so. The degree of occupation is usually high and 
the number of stages so few that only the first one can be regarded as initial. 
As a consequence it seems desirable to speak of a pioneer stage only in 
primary succession and to designate the opening stage of a secondary sere as 
the first or initial stage. 

Medial stages. — The general demarcation of these from initial stages has 
been sufficiently indicated above. They are characterized by a fairly uniform 
density, by well-developed dominance, and usually by the increasing abun- 
dance of humus, together with medium amounts of water. They consist of 
well-developed communities in which layers have begun to appear. The most 
characteristic life-forms are grasses and shrubs. Medial stages may best be 
regarded as including all the stages between initial and climax ones. In all 
seres but those with a forest climax, these are all the stages after the initial 
ones but the last. When succession ends in forest, it seems desirable to con- 
sider all the successive forest communities as climax stages, though only the 
last is the climax association. The number of medial stages is several in 
primary seres, and few, often only one or two, in secondary ones. In both, the 
term must be regarded as comparative and relating chiefly or solely to position 
in the sequence, since grassland stages are medial in a region with a forest 
climax, and climax in a region of climatic prairie. 

THE CLIMAX. 

Concept. — Every complete sere ends in a climax. This point is reached 
when the occupation and reaction of a dominant are such as to exclude the 
invasion of another dominant. It does not prevent the entrance of subordi- 
nates, and it is conceivable that a codominant might enter also, though no 
case of this is known. The climax marks the close of the general develop- 
ment, but its recognition is possible only by a careful scrutiny of the whole 
process. Duration is in no wise a guide, since even pioneer stages may persist 
for long periods, and medial stages often simulate a climax. The test of devel- 
opment is especially necessary in climax stages, i. e., those in which the domi- 
nants belong to the same life-forms as the climax dominant. It is not merely 
indispensable to trace and retrace the course of succession in a particular 



106 STABILIZATION AND CLIMAX' 

locality. It is also imperative to follow the development in all parts of the 
climatic region where dominants occur which are similar to the one supposed 
to be the climax. There is no field in ecology where it is so necessary to 
employ both intensive and extensive methods to secure permanent results. 
The reason for this is obvious when it is fully recognized that the climax 
formation is the clue to all development and structure in vegetation. 

Nature. — The fundamental nature of the climax and its significance in the 
life-history of a vegetation are indicated by the fact that it is the mature or 
adult stage of the latter. As stated elsewhere, the climax formation is the 
fully developed community, of which all initial and medial communities are 
but stages of development. The general behavior of the formation as a com- 
plex organism resembles very closely that of the simple organism, the indi- 
vidual. The recognition of the latter is so natural and necessary a prelude to 
the study of its development and organization that it is taken for granted. 
In like manner the recognition and limitation of climax formations is indis- 
pensable to a proper developmental study of vegetation. It is not at all the 
usual method of approach as yet, because its unique importance has not been 
generally recognized, but in the future much more attention must be paid 
to the climax stage if the problems of development and structure are to be 
clearly foreseen and solved. In fact, the study of succession in any climatic 
region should be begun by an intensive and extensive study of the adult 
organism, the development of which is to be traced. This is especially neces- 
sary in view of the complex nature of succession and the number of adseres 
and subseres that may occur in the development of any formation. The need 
of such a method of study is further emphasized by the fact that prisere and 
subsere are but reproduction processes of the formation and as such can 
be understood only by an understanding of the formation itself (plate 
13, a, b). 

Relation to succession. — The explanation of the universal occurrence of a 
climax in succession lies in the fact that the succession is reproduction. The 
reproductive process can no more fail to terminate in the adult form in vege- 
tation than it can in the case of an individual plant. In both instances it 
may fail under abnormal, i. e., unfavorable, conditions. The lack of light in 
dense thicket or woodland will prevent the maturing of herb or woody plant, 
as it will of aquatic and amphibious plants when too deeply submerged. An 
excess of water will have similar effects, while a deficit often suppresses the 
vegetative stages in large degree. The action of man or animals may keep 
the plant in an immature condition throughout its life history. While the 
response is usually more complex, the behavior of the formation is strictly 
comparable. Natural or artificial factors may hold it almost indefinitely in 
an imperfect condition of development, i. e., in practically any initial or 
medial stage, or may cause reproduction of little more than the adult stage 
alone. Man in particular may cause a developmental stage to become per- 
manent, or to recur so constantly that it appears to be fixed. 

The underlying causes of complete development of the formation are to be 
sought in the habitat, just as they are in the case of the individual. Favorable 
or normal water and light relations result in normal or complete development ; 
unfavorable or abnormal conditions cause suppression of part of the course. 
The significant difference lies in the fact that the reactions of the individuals 



CLEMENTS 







A. Climax prairie of Stipa ami Agropyrum, Winner, South Dakota. 
B. Climax forest of Psciulotxuf/a, Tsitjta, ami Thuja, Mount Rainier, Washington. 



THE CLIMAX 107 

as a community produce a cumulative amelioration of the habitat, a progres- 
sive improvement of the extreme, intrinsic to the continuance of development 
itself. In the case of heath, the production of "bleisand" and "ortstein" are 
unfavorable to further development, but such a consequence of reaction is 
wholly exceptional. Indeed, this hardly constitutes an exception, since the 
persistence of such conditions produces a climax. The climax is thus a product 
of reaction operating 1 within the limits of the climatic factors of the region 
concerned. The latter determine the dominants that can be present in the 
region, and the reaction decides the relative sequence of these and the selection 
of one or more as the final dominant, that is, as the adult organism. 

Kinds of climaxes. — The climatic formation is the real climax of the suc- 
cessional development. As has been seen, various agents may interpose to 
prevent complete development. The result is to produce apparent climaxes 
of greater or less duration. These depend absolutely upon the continuation 
or recurrence of the action which inhibits further development. They dis- 
appear as soon as the causative force is withdrawn, and the course of succes- 
sion resumed in consequence. Such apparent climaxes are always subordinate 
to the normal developmental or climatic climax, and may accordingly be dis- 
tinguished as subclimaxes. The application of this term is based upon the 
two-fold meaning of the prefix sub, of which the original sense is beneath 
or under, and the transferred meaning somewhat or rather. The subclimax is 
always below or before the climax proper in point of time, and actually 
beneath it in such coseres as those of peat bogs. Likewise it is subordinate 
developmentally, though in dominance and persistence it may resemble a true 
climax very closely. In addition to subclimaxes, which are constituted by some 
stage antecedent to the climatic formation, there may be distinguished poten- 
tial climaxes which are often subsequent. A potential climax is the actual 
climax of an adjacent region. It is called potential because it will replace the 
climax of the region concerned whenever its climate is changed. The potential 
climax of plains grassland is scrub if the rainfall is increased ; it is desert if 
the temperature is increased. As is later shown at length, potential climaxes 
stand in a zonal relation to a particular formation, and this relation is that 
of the sequence of successional stages. 

Subclimaxes. — Various causes produce subclimaxes. Such are (1) soil, (2) 
reaction, (3) competition, (4) migration barriers, and (5) man. In spite of the 
greatest difference in their action, they agree in preventing development by 
handicapping or destroying some stage, usually a climax one. Apart from 
plant reactions, such an influence is probably exerted by the soil only when 
it contains an excess of salt. In the Great Basin the climatic formation is 
that of the sagebrush (Artemisia tridentata) , but vast alkaline stretches will 
long be covered by Sarcobatus and Atriplex. As a consequence of their reac- 
tion these will yield theoretically to Artemisia in the course of time, and this 
seems to be actually taking place at the margins of the alkaline area. In the 
present state of our knowledge, however, it is impossible to be certain that 
this can ever occur in the heai't of the region withoiit a change of climate. 
Reactions which retard succession instead of promoting it are few, but they 
are of great importance. Thei'e seem to be but two of these, that of Sphagnum 
in accumulating water, and that of moor and heath in producing acids or 
other harmful substances. These reactions, together with the consequent pro- 



108 STABILIZATION AND CLIMAX 

duction of heath-sand and "orstein," appear to enable moor and heath to 
persist for a long time over vast areas. There is, however, some warrant for 
thinking that these subclimaxes are due wholly or partly to the action of man. 

The exact role of competition is more difficult to ascertain, but there can 
be little doubt that it is important and sometimes controlling in maintaining 
a grassland subclimax. This is said to be true of the Ceylon patanas by Pear- 
son, and it is also confirmed by evidence from the prairies and from moun- 
tain meadows. A subclimax due to barriers to immigration occurs whenever 
such final dominants as Picea or Fagus are prevented from spreading through- 
out a natural region. Thus Pinus and Quercus have formed or still form sub- 
climaxes in areas ultimately to be occupied by beech or spruce. In the valleys 
of the Missouri River and its tributaries in Nebraska, as elsewhere along the 
western margin of the Mississippi Basin, the forest is in a subclimax stage com- 
posed of Quercus, Hicoria, and Juglans. Further westward, the valley woodland 
is a subclimax formed by a still earlier stage composed of Populus and Salix. 

Subclimaxes due wholly or partly to the activities of man are numerous. 
Conspicuous causes are burning, clearing, and grazing. These produce sub- 
climaxes in a particular area by disturbance and destruction of the com- 
munity. This results in subclimaxes in adjacent areas in consequence of 
destruction of the source of migrules. Grassland areas are produced the world 
over as a result of burning and grazing combined, and they persist just as long 
as burning recurs. Woodland is frequently reduced to scrub by fire, and the 
scrub often persists wherever repeated fires occur. Even when fires cease with 
the settlement of a region, grassland and scrub subclimaxes persist for a long 
time because of the more or less complete removal of the forest. The clearing 
of the forest in connection with lumbering or cultivation may result in more 
or less permanent scrub. When clearing is followed by fire or grazing or by 
both, as is often the case, the scrub may be entirely replaced by grassland, 
which remains as a subclimax as long as the causes are effective, or may per- 
sist almost indefinitely in consequence of the removal of natural forest and 
scrub from the region. In the case of silvicultural activities, it is evident that 
any forest stage may be fixed as a subclimax, or that a new subclimax may 
be produced artificially by the planting of exotics. Similar modifications are 
possible in the treatment of natural grassland. The final climax in a grass- 
land region, such as that of the Great Plains, may be inhibited by fire or 
grazing. The area may remain for a long time in a grass subclimax, such as 
the Aristida consocies, or it may show an undershrub climax of Gutierrezia 
and Artemisia. 

Potential climaxes. — As has been stated previously, zones of vegetation 
indicate the changes of vegetation possible in consequence of a change of 
climate. This is fairly evident in the case of zones which correspond to marked 
differences in latitude or altitude, but it is equally true of other great zones, 
such as the prairie, plains, and interior basin of North America. These are 
all responses of vegetation to a progressive change in the controlling factors, 
as is true of the more striking zonation of ponds, streams, islands, etc. The 
regional zones are produced by the cumulative change of climatic factors in 
one direction, while the local zones are due to the gradual change of water- 
content, often in consequence of reaction. The latter are independent of 
climate to the extent that they exist beside each other, but they are only 



THE CLIMAX 109 

records of a development which conies increasingly under climatic control 
with every step away from the original extreme of soil conditions. The zones 
of a prairie lake are the result of the reaction control, or what might be called 
the habitat control, of succession, but the paramount part of climate in the 
development is shown not merely by its setting the usual climax limit, but by 
the fact that it can fix an earlier or later limit. Normally, the stages of inva- 
sion end with the outermost zone, since this is the climax in which the new 
area for development has been set, but a change of climate in the direction of 
greater rainfall or less evaporation would continue the development beyond 
prairie into woodland. The latter then becomes an intrinsic member of the 
successional sequence as recorded in the series of zones. 

Changes of climate. — A change of climate can not initiate succession 
except where extreme drouth or frost destroys essentially an entire plant com- 
munity. Practically no such instances are recorded for native vegetation, and 
such climatic changes as we know can only continue a sere already begun or 
bring it to a close in a stage earlier than the climax. Indirectly, changes of 
climate may result in new areas being produced by other agencies as a conse- 
quence of increased rainfall. When operating over long periods, they may 
produce profound changes of the flora, and hence alter the whole climax 
community and its development. The effect of climatic oscillations may be seen 
from year to year in the ecotone between two climatic associations. In short, 
the ecotone is largely a record of the effects of small variations of climate. If 
accumulated or allowed to act in one direction, the latter are sufficient to give 
the advantage to one of the contiguous associations. In the midst of the prairie 
region, the forest edge of the valleys yields in years of severe drouth, as in 
1893-1895, while in a series of years with unusual rainfall it advances visibly. 
If similar dry or wet conditions become permanent, the forest would gradually 
give way before the prairie, or the latter would disappear before the forest. 
The completeness of the replacement would depend upon the amplitude and 
the duration of the climatic change. All timberlines, especially alpine ones, 
show similar movements, and the latter can be recognized in all herbaceous 
ecotones, though with less readiness. When the change of climate favors meso- 
phytic conditions, the existing seres are continued by the addition of one or 
more stages dominated by higher life-forms. In the case of the prairie, the 
potential climaxes in this case are deciduous forest in the east, and scrub and 
pine woodland in the west. An efficient increase in rainfall might well bring 
these two together, and result in the prairie climax being replaced by a pine 
climax in the present plains area and a deciduous forest climax in the prairie 
area proper. It is far from improbable that something of the sort has happened 
in the past. Such a contact has actually occurred in the valley of the Niobrara, 
where Pinus ponderosa reaches its eastern limit just east of the one hundredth 
meridian, where it is met by Juglans, Vlmus, Tilia, and other members of the 
deciduous woodland (Bessey, 1887, 1896:109, Pound and Clements, 1900:322^. 
If the swing of climate results in decreased rainfall, the potential climax is 
found in the areas with a vegetation one stage more xerophytie than the 
existing climax. These are the crests and ridges on which the present climax 
has not yet established itself, or the secondary disturbed areas which are in 
the subclimax stage. The corresponding communities of Aristida, or of 
Gutierrezia-Artemisia, would probably become the climax vegetation, though 



110 STABILIZATION AND CLIMAX 

certainty is impossible since the present tendency over much of the prairie 
and plains area is favorable to scrub and woodland. 

Preclimax and postclimax. — The significance of potential climaxes is best 
seen in the case of mountain ranges which rise directly from the plains. Such 
are the Front and Rampart Ranges of Colorado. In these, the narrow zones 
stand out sharply, and the effect of possible changes of climate is demonstrated 
most clearly by east-and-west canons. On the north exposure of these a meso- 
phytic association may descend far below its horizontal limit and thus occur 
alongside of one which it would eventually replace if the rainfall were to 
increase generally. On south exposures of the canon, the more xerophytic 
communities ascend far above their usual limit, and place themselves in con- 
tact with the normal climax which would yield to them in case of decreased 
rainfall. As a consequence it becomes possible to recognize two kinds of 
potential climaxes. The one indicates what will happen if a change of climate 
results in increased water-content, thus emphasizing the normal reaction in 
the sere. It continues the development by replacing the climax and it may 
be termed the postclimax (Gr. 7ros; Lat. post, after). The other foreshadows 
the climatic change which reduces the water-content, and thus sets a lower 
limit to the increase of the holard by reaction. As a consequence, develop- 
ment would cease before reaching the climax proper, and the potential com- 
munity, which would now become the actual climax, may be called the pre- 
climax (Gr. 7t/di; Lat. prae, before). Thus, every climax area or formation 
is in contact with one or more climax areas which bear the relation of pre- 
climax and postclimax to it, and are in a more or less complete zonal series 
with it. Subclimaxes are practically always preclimaxes (plate 14, a, b). 

Changes of climax. — As already noted, the climax may change in conse- 
quence of a single efficient variation of climate or of the development of an 
essentially new flora as the outcome of long-continued evolution due to climate. 
In addition, the climate may show a cumulative change, or it may exhibit great 
alternations, such as those indicated in Blytt's theory (1876). Both of these 
phenomena were associated, it would seem, with the glacial period. It is 
not difficult to surmise the behavior of the successive climax formations in the 
face of the oncoming ice. A gradual invasion must have produced preclimaxes 
in all of the seres actually in development, before it overwhelmed each climax 
area. The area just south of the final limit must have developed a series of 
preclimaxes, ending in arctic tundra. Each recession of the ice must have 
changed serai climaxes into postclimaxes, and each new advance would cause 
the existing seres to terminate in preclimaxes. The final withdrawal of the 
ice would give new areas for colonization by an arctic flora, and hence a new 
arctic climax, while the original arctic climax about the southern edge would 
yield to the postclimax of heath or aspens and conifers just south of it. In a 
similar manner, the postclimax of deciduous forest would replace the conifers, 
and these again a new arctic climax of which they were the potential climax. 
Finally, when climatic equilibrium was established, the arctic zone south of 
the original ice would have had three or four successive climaxes, and the 
number of climaxes would decrease by one for each zone to the northward. 
For any particular period, each climax zone may have had a sequence of 
seres, i. e., a cosere, all ending in the actual climax. In the case of the alter- 
nating wet and dry climates which followed the glacial period, the postglacial 



CLEMENTS 




&?$r0S^wl*™*'-'* 




A. Postclimaxes of scrub (Shepherdia, Amelanohier, etc.) and of woodland (Z7tmiu 
Fraxiiuts, Querents macrocwpa) in prairie climax, Gasman Coulee, Minot, X. D. 
B. Sagebrush preclimax (Artt misia tridentata) and Pinits ponderosa climax. Bates 

Park, Colorado. 



THE CLIMAX 111 

deposits seem to furnish convincing evidence of a sequence of climaxes derived 
from postclimaxes. Thus, the arctic climax, the Dryas association, was suc- 
ceeded by an aspen climax, the latter by a pine climax, this by an oak climax, 
and the oak by the beech climax of to-day. The sequence apparently corre- 
sponds with the gradual amelioration of temperature in large degree, and is 
concerned with changes of rainfall only in so far as they favored or hindered 
the growth of Sphagnum, and thus caused successive seres, the climaxes of 
which were preserved by being embedded in the peat-bog. 



VII. STRUCTURE AND UNITS OF VEGETATION. 
DEVELOPMENT AND STRUCTURE. 

Relation. — Development is the process by which, structures are fashioned. 
This is as true of the climax formation as it is of the mature individual. Each 
is a climax stage with characteristic structure produced by development. 
Moreover, both formation and plant exhibit structures in the course of growth. 
Some of these are retained and contribute to the final form, others are tran- 
sient and disappear completely after they have fulfilled their function. In 
the case of the individual, most of the structures persist and play their part 
in the work of the adult. That this is not necessarily true is shown by the 
usual behavior of cotyledons and stipules. It is also seen in the complete or 
partial disappearance of leaves and stems, and especially in the fate of flower 
parts. From the nature of the plant community, the earlier structures are 
replaced by later ones, though they may persist in some measure, especially 
in secondary seres. Finally, the development of both formation and plant 
is a series of responses to the progressive change of basic factors, which not 
only control the course of development but determine also its culmination 
in the adult. 

Kinds of structure. — The nature of succession as a sequence of commun- 
ities from extreme to medium conditions determines that its major and uni- 
versal expression in structure will be zonation. This is convincingly shown in 
water seres, where the zonation from the center to the margin, due to water 
relations, is repeated in the zones or layers which succeed each other as the 
center is shallowed. In essence, the zones of the margin move successively over 
the surface, and are recorded as superimposed zones in the peat. Whenever 
conditions change abruptly instead of gradually, zonation is replaced or 
obscured by alternation. The latter is strikingly evident in extensive com- 
munities which are disturbed here and there by denuding agents. The result- 
ing bare areas give rise to secondary seres, the stages of which when viewed as 
static communities seem to be unrelated to the circumjacent vegetation. As 
a matter of fact, they are merely incomplete expressions of successional zones, 
as is readily observed when the denuding force has operated unevenly over 
the entire area. The layers of forest and grassland are zonal structures which 
are more or less evidently connected with succession. The seasonal aspects 
of vegetation, though recurrent, are also developmental, and often stand in 
intimate relation to layering. 

Zonation. — Zonation is the epitome of succession. Zones are due to the 
gradual increase or decrease in a basic factor, typically water, from an area of 
deficiency or excess. Successional stages are produced by the slow change of 
a bare area from one of deficiency, e. g., rock, or one of excess, water, to more 
or less medium conditions. In the case of water, for example, the bare area 
of excess is the starting point for the series of zones, as it is for the series of 
stages. In short, zones are stages. This fact has been generally understood 
in the case of zones around water bodies, in connection with which it was first 
clearly stated by De Luc (1810:140) in the following sentence: 
112 



DEVELOPMENT AND STRUCTURE 113 

"The succession of the different zones, from the border of the water 
towards the original border of sand, represents the succession of changes that 
have taken place through time in each of the anterior zones, so that in propor- 
tion as the reeds advance, new zones are forming behind the advancing reeds 
on the same places which they thus abandon. ' ' 

It has not been recognized that it reveals a basic and a universal principle. 
It is just as true of the climax formations of a continent, with zonal disposi- 
tion in accordance with latitude and altitude, as it is of the zones of a lake or 
river or those of hill or ridge. The latter are zones of actual succession, the 
stages of existing seres ; the former are zones of potential succession, and indi- 
cate the further stage of development in the event of a change of climate. 
Both are possible stages of the same great development, and are equally con- 
trolled by the gradual change of conditions, though the change in one case is 
climatic, in the other edaphic. 

~3~Sed$elarid 



\v^^ V-,^^1 Submerged 

Fig. 3. — Schematic representation of the development of the hydrosere, showing 
identity of zones and serai stages. Straight lines indicate zones, and dotted 
ones their extension across the pond as the latter is shallowed. 

The intrinsic relation between zones and stages is best proved by the zona- 
tion about water, on account of the relatively rapid decrease in water-content. 
It is equally well shown by areas with rapid increase about a dry center, such 
as islets in the lakes of arid regions, but these are relatively infrequent. Ponds 
and streams with gently sloping margins often show at one time a complete 
series of zones representing the successive stages of development to the climax 
association. The relation of these zones in time is clearly demonstrated by 
projecting them across the water center, as is seen in figure 3. Such a pro- 
jection occurs by degrees during the course of development, until the center 
is occupied in the proper sequence by every stage from the submerged to the 
climax community. The proof of this is found in practically all peat deposits, 
but especially in those where the development has been gradual and complete. 
The actual extension of the various zones over the water-body or a portion of 
it occurs when a pioneer or subpioneer community, such as a Sphagnetum, 
develops as a floating mat which becomes anchored at the bottom or the side. 
Such seres furnish the complete demonstration of the identity of zones and 
stages, and also serve to emphasize the fact that every zone has a temporal 
as well as a spatial relation, and hence is the result of development. 

The filling by reaction of a pond or lake with a uniformly shallow bottom 
and abrupt banks is of especial significance in correlating climatic zones with 
edaphic ones. In such ponds, which are typical of the prairie region, the 
spatial relation is over-emphasized, the temporal relation obscured. It not 
infrequently happens that there is complete unconformity between the pond 
community and the climax vegetation in which it occurs. In a word, the 
usual zones are lacking, since there is no gradual shallowing of the water 
toward the climax area. The consequence is that each stage, instead of form- 



114 STRUCTURE AND UNITS OF VEGETATION 

ing a zone as normally, occupies the whole area for a longer or shorter period 
in the usual sequence of succession. It shows no organic connection with the 
climax association until the development is completed, and in itself furnishes 
no direct evidence of succession. In fact, when the area occupied by such a 
community is large, it simulates a climax association. Apart from its resem- 
blance to other communities whose development is known, its real nature can 
be ascertained only by actually following the sequence of stages or by probing 
the deposits of plant remains. As a matter of fact, water seres are too well 
understood to cause difficulty in this connection, and the illustration is of 
importance only because it clarifies the developmental relation of the great 
climatic zones. The latter also seem to have no successional connection, but 
this is only a seeming, as has already been indicated under potential climaxes. 
An effective swing of climate at once places each climax area in successional 
articulation with an adjoining one, and reveals its essential nature as a 
developmental zone. It has been shown above that the climax changes of the 
glacial and the postglacial periods not only transformed climax zones into 
successional stages, and the reverse, but also that it left a record of such zonal 
stages in the layers of peat-bogs. 

The zonation of hills and ridges in the prairie formation is typical of the 
relation between structure and development. Owing to the more or less 
uniform nature of grassland, distinct zones are rarely evident, but a careful 
scrutiny shows that a majority of the societies are in zonal relation. Exposed 
rocky crests bear lichen colonies, about which are xerophytic communities 
of Lomatium, Comandra, Meriolix, and others. Middle slopes are occupied 
by more mesophytic species, such as Astragalus, Erigeron, Psoralea, etc., and 
the bases and ravines by meadow species, especially grasses and sedges. In 
some of the ravines small marshes or ponds develop and add one or more 
zones to the series, while in others, thickets of Salix, Rhus, or Symphoricarpus 
appear, making possible the invasion of woodland herbs, and the occasional 
entrance of Populus or Fraxinus. Imperfect as the zonation of the prairie is, 
it furnishes an indubitable record of the development of the association from 
xerophytic ridge communities on the one hand and from ravine communities 
of meadow and marsh on the other. In addition, the ravine thickets suggest 
the fate of the prairies when confronted by an increase of rainfall, or when 
artificial barriers to the spread of woodland are withdrawn. 

The zonation of fringing forests is perhaps best seen in prairie and plains 
regions, owing to the fact that the decrease of water-content from the edge 
of the stream to the dry grassland takes place rapidly. In addition, there is 
a similar rapid decrease of humus and increase of light intensity. In many 
places actual zones are lacking or fragmentary, owing to local conditions,- in 
others, the complete series may find expression. In the Otowanie Woods 
near Lincoln, Salix, Populus, and Vlmus either indicate or constitute narrow 
zones from the water to the oak-hickory climax. In the direction of the grass- 
land, Fraxinus, Rhus, and Symphoricarpus constitute as many zones in more 
level areas, while on steep slopes only a narrow band of thicket may occur, or 
the scrub oak (Quercus macrocarpa) may gradually dwindle into a shrub but 
a foot or two high. 

Relations of climax zones. — Like all zones, climatic ones are due to a 
gradual change in the amount of one or more controlling factors. They differ 



DEVELOPMENT AND STRUCTURE 115 

from edaphic zones in the fact that the plant reactions affect the general 
climate but little. They are in consequence relatively permanent, and disclose 
their successional relationship only as a result of pronounced climatic changes. 
The zonal arrangement of climax associations and their consociations is pro- 
duced by the gradual decrease in water and temperature from an area of 
excess. The effect of reduced temperature is found in the direction of the 
poles, and produces east and west zones. The effect of diminishing rainfall 
operates from coast to interior, and is recorded in zones which run north and 
south. The two are necessarily superimposed, and the final expression in terms 
of structure is further complicated by the influence of mountain ranges and 
large interior bodies of water, such as the Great Lakes. Mountain ranges may 
not only disturb the primary climatic zones, but they also present new regions 
of relative deficiency and excess, and consequent zonation. 

Direct evidence of the successional relation of climax zones, such as is uni- 
versal for edaphic zones, is not abundant. There are, however, several 
sources of conclusive proof of their essential developmental connection. The 
most important evidence is that furnished by peat-bogs and tufa deposits, 
which bear witness to successive climax stages due to change of climate. The 
similarity of these to zoned climax communities of to-day leaves no doubt of 
their zonation, which is also attested by the fact that the zones of to-day about 
water-bodies are recorded in superimposed layers of plant remains. Further 
evidence is afforded by the vegetation of canons. The well-known fact that 
the local climate of the north and south exposures is very different has already 
been dwelt upon. The result of this difference is to produce in miniature the 
effect which a general climatic change would cause over the whole mountain- 
slope. A change in the direction of greater heat or dryness would tend toward 
the xerophytic preclimax of the south exposure, while the opposite change 
would give rise to the postclimax of north exposures. Indeed, the behavior 
of such consocies as that of Pinus ponderosa is direct proof of the develop- 
mental nature of climax zones. At lower altitudes it forms a xerophytic 
climax over a vast stretch of the Rocky Mountain region ; at elevations 2,000 
to 3,000 feet higher it is the subfinal stage in the development of spruce forest. 
In other words, its spatial or zonal relation as a mountain climax to the sub- 
alpine spruce climax indicates its precise successional relation in the develop- 
ment of the latter. 

The bilateral zones of river valleys also furnish evidence of the potential 
development consequent upon climatic change. This is especially true where 
the valley lies in the direction of decreasing rainfall, as is true of the Niobrara, 
Platte, Republican, and others. The result is not only that forest, scrub, and 
grassland are brought into the closest zonal juxtaposition, but also that there 
is a gradual shifting of the consocies, as the edaphic conditions of the river- 
bottom are modified by an increasingly arid climate. The developmental 
series previously indicated for the Otowanie "Woods, namely, Rhus, Fraxinus, 
and Querciis-Hicoria, often with other consocies also, becomes a climatic series 
with exactly the same sequence from moist to dry conditions. Finally, the 
broad ecotones or transition areas between climax communities are clear 
indexes of the effect of climatic swings. They are mixed communities, and 
correspond closely to the mixture of two contiguous stages, i, <>.. a mictium. 
in the course of succession. 



116 STRUCTURE AND UNITS OF VEGETATION 

Significance of alternation. — Alternation is the consequence of disturbed or 
incomplete zonation. Sueh areas produce alternes, which it now seems can 
always be related to more primary zones. This has already been shown in 
the case of the alternes of canons, which are only the upward or downward 
extension of zones. Wherever the conditions which control zonation are dis- 
turbed, alternation is produced, just as is the case whenever the conditions 
for a particular zone occur abruptly or locally. An excellent example of the 
latter are the extra-regional pockets of Celtis or Symphoricarpus, described by 
Pool (1914) in the sand-hills of Nebraska. These are fragments of consocies, 
whose zonal relations are evident only where climatic conditions permit the 
development of forest. Similar detached thickets of Cercocarpus occur in 
the "Wildcat Mountains of western Nebraska. Their proper relation can be 
understood only by a study of Cercocarpus as a consocies of the foot-hills of 
Colorado and Wyoming, where it is associated with Quercus, Rhus, and other 
shrubs. To Quercus and Rhus trilobata it bears a distinctly zonal relation, 
since it is the most xerophytic of the three, and consequently occupies knolls 
and ridges. The foot-hills, however, are so much dissected and bear so many 
outcrops of rock that the fundamental zonation is greatly interrupted, and in 
some cases thorough examination alone will disclose the fact that the numerous 
alternes are actually fragments of zones. The rolling character of the prairies 
has a similar effect. Ravines, gullies, and ridges of varying extent and rank 
are so numerous that zonation is often completely obscured and can be 
revealed only by tracing the distribution of characteristic species. This effect 
is enhanced by the many kinds of exposure and the ever-changing angle of 
slope and their effect upon both migration and ecesis. 

Developmental relation of layers. — Fundamentally, layers are zones 
related to the decrease in light intensity from the primary layer toward the 
soil, though the increasing shade is really the reaction of the constituent 
species. Layering differs from zonation in being vertical instead of lateral and 
in giving a correspondingly complex structure to the community. Thus, while 
the developmental relation of layers is certain, it is not obvious. It is most 
evident in the layers of submerged, floating, and amphibious plants in water, 
since these are of course so many developmental stages and are associated 
only in mictia. The most typical development of layers is in forest, and this 
alone need be considered, since the less complete layering of grassland, herb- 
land, and scrub is fundamentally similar. In the forest with a complete set 
of layers the latter indicate in a general way the sequence of life-form stages 
from the ground-layer of mosses and lichens through herb, grass, and shrub 
layers to the primary layer of trees. The species correspondence of layers and 
stages is usually slight or none, owing to the great difference in light intensity 
after the forest is established. However, a few species adapt themselves so 
readily that they persist for some time during the forest climax, and play a 
recognizable part in the constitution of layers. Such a result is indicated by 
the reciprocal fact that some species of the forest undergrowth are able to 
persist after the trees have been removed. In the spruce forests of the Rocky 
Mountains, Opulaster often persists to become the dominant species of the 
shrubby layer. In the final maturing of the spruce forest the number of 
layers is directly dependent upon the increasing density of the crown, and 
hence serves as a ready index of the degree of maturity, i. e., of development. 



THE UNITS OF VEGETATION 117 

The disappearance of the layers beneath the primary one follows the life-form 
sequence, but in the reverse order, the shrubby layer disappearing first, the 
bushes next, then the tall herbs, and last of all the ground herbs, the mosses 
and lichens remaining as the final remnant of the layered condition. 

Relation of seasonal aspects. — In forest and thicket, aspects are due to the 
occurrence of societies at times when light conditions are most favorable. 
The prevernal aspect of deciduous woods is characterized by a ground-layer 
of species which develop before the woody plants unfold their leaves and 
before the other layers have appeared. In general, the herbaceous societies 
bloom and give character to the different layers in the order of height, so that 
the seasonal development recapitulates in some degree the succession of life- 
forms. The seasonal aspects of the prairie show a somewhat similar relation, 
though the cause is found in the water and heat as well as the light relation. 
The prevernal and vernal societies and clans are composed of low-growing 
herbs, such as Anemone, Astragalus, Lomatium, Viola, etc., which correspond 
to a ground-layer. The summer societies are tall-growing, and often allow 
the development of one or two layers beneath them. The serotinal aspect 
is likewise characterized by societies of tall plants, with at least partial secon- 
dary layers. Apart from the relation of the prairie aspects as layers, there is 
also a general developmental relation in that the conditions are nearest like 
those of meadow in the spring, and are most typical of the prairie in summer 
and autumn. 

THE UNITS OF VEGETATION. 

HISTORICAL SUMMARY. 

The formation concept. — Although the detailed consideration of the struc- 
ture of vegetation is reserved for another volume, it is desirable to consider 
here the chief concepts of the formation. No term has had a more varied 
experience or a larger variety of uses. Efforts to discard it have been futile, 
and attempts to definitize it of little avail. Like all plant structures, it is 
the outcome of development, and hence can not be absolutely delimited. The 
difficulties in its definition and use seem to have arisen from a failure to 
recognize its developmental character, as is shown later. As is true of all 
biological concepts, its first significance was necessarily superficial and incom- 
plete. But the concept has broadened and deepened until, with the adoption 
of the developmental idea, it includes the whole group of relations between 
the basic unit of vegetation and its habitat. The history of the formation 
concept is the history of this process of refinement and definitizing. 

Grisebach's concept of the formation. — As is generally known, Grisebach 
(1838:160) was the first to use the word "formation": 

"The first method, the employment of which even a very superficial knowl- 
edge of a region makes possible, is based upon the physiognomy of vegetation. 
upon the grouping of individuals in the mass. I would term a group of plants 
which bears a definite physiognomic character, such as a meadow, a forest, 
etc., a phytogeographic formation. The latter may be characterized by a 
single social species, by a complex of dominant species belonging to one 
family, or, finally, it may show an aggregate of species, which, though of vari- 
ous taxonomic character, have a common peculiarity ; thus, the alpine meadow 
consists almost entirely of perennial herbs. In a general account of the forma- 



118 STRUCTURE AND UNITS OF VEGETATION 

tions of a flora, it would be necessary to indicate the character plants and to 
determine the species to which they owe their physiognomic features, which 
are in no wise subjective. This is a task especially to be recommended to 
travellers, since it can be carried out easily and thoroughly. These formations 
repeat themselves everywhere in accordance with local conditions, but they 
find their absolute, their climatic limits with the natural flora, which they con- 
stitute. Just as far as forests of Pinus silvestris or heaths of Calluna vulgaris 
extend, just so far does one find himself in the region of the middle European 
flora. Even if a single species of one flora pass into another, a dominant 
species of a group does not appear at the same time in two floras. Every for- 
mation, whose character and components are indicated with distinctness, con- 
forms to the limits of its natural flora. ' ' 

From the above, it is obvious that Grisebach's conception of the formation 
was essentially if not wholly physiognomic. This was also true of the idea 
underlying Humboldt's use (1807:17) of the term association. While it is 
possible to find much harmony between the use of this term by Humboldt 
and by many modern writers, it seems obvious that Humboldt and Grise- 
bach meant practically the same thing by their respective terms. Indeed, 
Moss (1910:21, 28) has already pointed out this fact in the case of both 
terms. 

Drude's concept. — Drude (1890:28) has criticized Grisebach's concept 
and has insisted upon the necessity of considering the flora as of more impor- 
tance than the physiognomy: 

"Grisebach's definition of the formation must be taken in its entirety. It 
appears correct to regard the ' groups of plants, which bear a definite physiog- 
nomic character' as classes of formations. The occurrence of these and their 
extent permits one to distinguish the great vegetation zones of the earth, but 
they throw no light upon the question of floristic. Definiteness can be 
secured only by means of the latter, particularly if one considers that the 
special physiognomy is due simply to the dominant species, and without 
inventing a special physiognomic system. Therefore the essential task, in 
order to secure a general survey of the formations of a flora, is to determine 
their dominant species, and, one may add, to study their local conditions. 
Therefore I view the concept of formation in this later sense with reference to 
a particular flora. 

"Hence, I regard as a vegetation formation, within the limits of a definite 
phytogeographic flora, each independent closed chief association of one or 
several life-forms, the permanent composition of which is effected by the 
definite conditions of the habitat, which keep it distinct from the adjacent 
formations. ' ' 

Drude here clearly assigns a basic role to the habitat, but his actual delimi- 
tation of formations is based primarily upon floristic. He (1896:281, 286) 
further emphasized the necessity of taking the habitat into account in deter- 
mining formations: 

"The division of the vegetative covering appears to be determined by the 
arrangement of definite habitats, and coincides with the alternation of the 
principal plant communities, in which the physiognomic character of the land 
lies hidden. These concepts are designated as vegetation formations, whicih 
are the botanical units of the vegetative covering of the earth. . . . Every 
independent chief association which finds a natural end in itself and which 
consists of similar or related life-forms in a habitat with the same conditions 



THE UNITS OF VEGETATION 119 

for existence (altitude, exposure, soil, water) is a vegetation formation. It is 
assumed that an essential change can not occur on the site of such a com- 
munity without external changes : the community is ' closed. ' ' ' 

Clements's concept. — Clements (1905:292) placed particular emphasis on 
the habitat as determining the formation concept and as affording a more 
accurate basis for recognizing and delimiting formations. 

"In vegetation, the connection between formation and habitat is so close 
that any application of the term to a division greater or smaller than the habi- 
tat is both illogical and unfortunate. As effect and cause, it is inevitable that 
the unit of the vegetative covering, the formation, should correspond to the 
unit of the earth's surface, the habitat. This places the formation upon a 
basis which can be accurately determined. It is imperative, however, to have 
a clear understanding of what constitutes the difference between habitats. 
A society is in entire correspondence with the physical factors of its area, and 
the same is true of the vegetation of a province. Nevertheless, many societies 
usually occur in the same habitat, and a province contains many habitats. 
The final test of a habitat is an efficient difference in one or more of the direct 
factors, water-content, humidity, and light, by virtue of which the plant cover- 
ing differs in structure and in species from the areas contiguous to it. This 
test of a formation is superfluous in many cases where the physiognomy of 
the contiguous areas is conclusive evidence of their difference. It is also 
evident that remote regions which are floristically distinct, such as the 
prairies and the steppes, may possess areas physically almost identical, and 
yet be covered by different formations." 

This concept of the formation recognized both the physiognomic and 
floristic sides, but assigned the chief value to the habitat because of its funda- 
mentally causative character. The habitat was regarded as something to be 
measured and studied exactly, with the object of determining the causes of 
the development and structure of communities, and hence arriving at the 
real limits of the formation and its divisions. 

Moss's concept. — Moss (1907:12) was the first to take development into 
account in determining formations: 

"A plant association in which the ground is carpeted in this sparse man- 
ner, with patches of bare soil here and there, is spoken of as an open associa- 
tion. An open association represents an early stage in the succession of 
associations which finally lead to a closed association, when the ground is 
fully occupied by one or a very few dominant plants, and a period of stability 
of vegetation has been reached. The series of plant associations which begins 
its history as an open or unstable association, passes through intermediate 
associations, and eventually becomes a closed or stable association, is, in this 
paper, termed a plant formation." 

Moss (1910:35, 36) states in a later paper that he: 

"Followed many previous authors in delimiting formations primarily by 
habitat, and then subdividing the formations into associations. This writer 
laid stress upon the succession of plant associations, especially on the succes- 
sion of associations within the same formation. It is necessary to distinguish 
the series of associations within a whole succession, that is, the succession 
from one formation to another, and the succession of associations within one 
and the same formation; and Moss enunciated a statement of the formation 
from the latter point of view." 



120 STRUCTURE AND UNITS OF VEGETATION 

As indicated above, Moss's intention was to base formations primarily upon 
habitat. Since he regarded the latter chiefly in the light of soil relations, it 
was inevitable that he should group together in the same formation the serai 
associations, such as forest, scrub, and grassland, which are due to efficient 
differences of light and water-content. As a consequence, Moss's concept of 
the formation was only incidentally developmental, and his actual formation 
is a different thing from the climax formation. While the latter is the out- 
come of development, it consists of two or more related climax associations, 
and not of a climax association plus two or three antecedent developmental 
associations. The views of Moss in regard to the formation were adopted by 
Moss, Eankin, and Tansley (1910) and by Tansley (1911). 

Schroter's concept. — Schroter (1902:68) has traced the outlines of a topo- 
graphic-physiognomic system of formation groups. The unit of this system 
is the local association, which characterizes a definite locality of uniform 
habitat. The diagnosis of the formation unit comprises (1) locality, (2) habi- 
tat, and (3) plant-cover, (a) physiognomy, (&) life-forms, (c) list of species. 
A formation comprises all the association types of the entire earth, which 
agree in their physiognomy and ecological character, while the floristic is 
immaterial. The series of units is as follows : 

I. Type Vegetation type Grassland. 

C Formation group Meadow. 

II. Formation . . -J Formation Dry meadow. 

(__ Subformation Alpine dry meadow. 

{Association type Nardetum. 

Subtype Nardetum with Trifolinim. 

Facies Nardetum (Nardus dominant) . 

Local association. Nardetum at Gotthard. 

In contrast to Schroter's topographic-physiognomic concept, Brockmann- 
Jerosch (1907:237) considers the first task in the study of plant communi- 
ties to be their limitation and description upon a physiognomic-floristic basis. 
The author expressly refrains from defining his concept of formation and 
association, but the essence of it is readily gained from the argument. In 
spite of the difference of emphasis upon habitat and floristic, the viewpoints 
of Schroter and Brockmann are very similar. They both accept the "pigeon- 
hole" concept of the formation proposed by Warming and justly criticized 
by Moss. 

Gradmann's concept. — Gradmann (1909:97) has also emphasized the im- 
portance of floristic at the expense of other criteria: 

' ' Since the physiognomic and ecological viewpoints have been shown inade- 
quate for a botanical distinction, there remains the sole possibility of ground- 
ing formations upon their floristic composition- In fact, the floristic method 
is the only one which can be completely carried out in a monographic treat- 
ment of formations. Many a well-marked and natural formation can be dis- 
tinguished in no other way than by its floristic composition. On the other 
hand, every formation determined by physiognomic characters can be circum- 
scribed just as well floristically. At the most one thereby obtains somewhat 
smaller units, which is by no means unfortunate, since it indicates nothing 
else than greater accuracy. As a consequence, floristic studies are always a 
substitute for pure physiognomic or ecological viewpoints, but the converse is 



THE UNITS OF VEGETATION 121 

not true. Moreover, the floristic method has the advantage of being purely 
analytical and hence highly objective. It is independent of physiological 
theories and does not presuppose a knowledge of causal relations, but leads up 
to it. It permits one to reckon with habitat and adaptations, as well as with 
unknown factors ; it proposes problems, and it stimulates to new investigations 
and advances. Through refining this method of determining the controlling 
factors by means of floristic agreements and contrasts, one can certainly 
obtain much insight into the factors of plant life, which have heretofore been 
overlooked. Thus, while the floristic analysis stands out as the most exact, 
most objective and most fruitful, and indeed as the sole universally applicable 
expression of the formational facts, yet it in no wise excludes the considera- 
tion of other viewpoints, but on the contrary encourages their use. Nothing 
stands in the way of adding to the floristic characterization a thorough analy- 
sis and description of the environic, physiognomic, ecologic, phytogeographic, 
and developmental relations. The chief emphasis must fall upon these funda- 
mental investigations and for these the floristic has only to furnish a basis 
free from objections. But such viewpoints can not serve for the limitation of 
formations; moreover, there is nothing to be gained from dubious and arbi- 
trary compromise. I hold therefore that we must universally recognize the 
floristic composition not merely as an important, but much rather as the basic 
and decisive criterion for the recognition of plant formations." 

Wanning 's concept. — Warming (1895) assigned to the habitat the chief 
value in determining plant communities. As he rejected the term "forma- 
tion, ' ' however, it is impossible to obtain his concept of the formation at this 
time. In the second edition of his pioneer work (1909 :140), he expresses the 
concept as follows: 

"A formation may then be defined as a community of species, all belonging 
to definite growth forms, which have become associated together by definite 
external (edaphic or climatic) characters of the habitat to which they are 
adapted. Consequently, as long as the external conditions remain the same, 
or nearly so, a formation appears with a certain determined uniformity and 
physiognomy, even in different parts of the world, and even when the con- 
stituent species are very different and possibly belong to different genera and 
families. Therefore — 

"A formation is an expression of certain defined conditions of life, and is 
not concerned with floristic differences. 

"The majority of growth forms can by themselves compose formations or 
can occur as dominant members in a formation. Hence, in stibdividing the 
groups of hydrophilous, xerophilous, and mesophilous plants, it will be 
natural to employ the chief types of growth forms as the prime basis of classi- 
fication, or, in other words, to depend upon the distinctions between treea, 
shrubs, dwarf -shrubs, undershrubs, herbs, mosses and the like." 

Moss (1910:39) has criticized Warming's concept of the formation, which 
treats the latter as a subjective group comprising all associations of like 
physiognomy. He considers that: 

"Warming has given the concept an unfortunate bias, and that his view is 
sufficiently at variance with historical and present-day usage to demand some 
examination of his treatment of this unit of vegetation. Confusion is apparent 
even in Warming's summary statement of the formation. Instead of a single 
fundamentum divisionis, Warming puts forward two tests of the formation, 
namely, definite plant forms ('growth-forms') and definite characters of the 



122 STRUCTURE AND UNITS OF VEGETATION 

habitat. It is not clear, either from his definition or from his general treat- 
ment of formations, what Warming precisely means by the term 'definite 
growth- f orms. ' In any case, the definition is defective, as plant form is not 
necessarily related to habitat : and therefore the two tests put forward in the 
one definition will frequently yield contradictory results. "Warming (p. 232) 
insisted that a salt marsh characterized by suffruticose Salicornias 'must be 
set apart from' salt marshes characterized by herbaceous Salicornias 'as a 
separate formation' merely because the plant form in the two cases is differ- 
ent. Such paradoxes occur throughout the whole of Warming's book; and 
indeed this Janus-like 'formation' is inevitable if plant form is to be allowed 
to enter into competition with habitat in the determination of formation. 
Warming's view might find some justification if definite plant forms were 
invariably related to definite habitats ; but it is quite certain that this is not 
the case. For example, on salt marshes in the south of England, it is no 
unusual thing to find associations characterized (a) by herbaceous species of 
Salicornia, (b) by suffruticose species, and (c) by a mixture of these. To 
place these associations in separate 'formations,' however, simply because of 
the different nature of the plant forms, is to reduce the study of formations 
to an absurdity." 

The foregoing criticism is as valid from the developmental viewpoint as 
from that of the habitat. It brings out in clear relief the fallacy of using a 
single basis for the recognition of formations, as is the usual method in most 
systems, notwithstanding statements to the contrary in defining the concept. 
There is of course no real contradiction between habitat and physiognomy, in 
spite of the fact that two or more life-forms may appear in the same habitat, 
and that the same life-form may recur in widely different habitats. The error 
lies in assuming that all species must make the same structural responses to 
a habitat, and that the general character of the life-form necessarily indicates 
its actual response. Nowhere in the field of ecology is there a more striking 
confirmation of the fact that development is the sole clue to follow through 
the maze of apparent and real, of superficial and basic relations between 
habitat, floristic, and physiognomy. 

Moss (1910:39) commends Warming for adopting, in connection with the 
division of the formation into associations, "a view which has forced itself 
on the minds of nearly all close students of vegetation." This is the view of 
Cowles (1899:111), in which he regarded the relation between the formation 
and association as similar to that of the genus to the species. The wording 
of Cowles 's statement is as follows: "One might refer to particular sedge 
swamp societies near Chicago, or to the sedge swamp formation as a whole; 
by this application, formation becomes a term of generic value, plant society 
of specific value." It is an open question whether the relation of particular 
local associations to an actual floristic entity is not really intended. In any 
event, it seems clear that there was no expressed intention of building up an 
artificial concept like the genus by placing in it all the swamp associations of 
the entire globe. Yet this is a legitimate if not necessary assumption from 
this comparison, and it is illogical to commend the acceptance of the principle 
and to object to the application of it. In the meaning of formation as used by 
those who regard it as a definite entity, the sole relation of the genus to the 
species that is shown by formation and association is that of a division and 
its subdivision. 



THE UNITS OF VEGETATION 123 

Negri's concept. — Negri (1914:33) has advanced a novel concept of for- 
mation and association, in accordance with which they become merely differ- 
ent viewpoints of the same thing : 

"We term formation this vegetation considered in the complex of its bio- 
logical relations, but not in its floristic composition; and understood in the 
totality of its individuals and in all the secondary variations of composition, 
of arrangement and of frequence, which it undergoes during the persistence 
of a physiographic unit of essentially unchanged edaphic conditions. (33) 
The formation is the physiognomic and ecologic expression of the association, 
as the biologic form is the physiognomic and ecologic expression of the species. 
(41) To the formation — biologic term — corresponds exactly the association — 
floristic term." (44) 

The adoption of this concept would result in the loss of the one point upon 
which practically all ecologists are in agreement, namely, the subordinate 
relation of the association to the formation. There can be no question of the 
need of physiognomic, ecologic, and floristic viewpoints of the formation, but 
their real values and significance appear only as they are considered in rela- 
tion to development. The author's failure to understand the fundamental 
nature of the formation as an organism with its own development is further 
indicated by his comment upon climax formations. (42) While it is quite 
possible to give the formation a different name for each of the four criteria, 
viz., development, physiognomy, habitat, and floristic, it is clearly inadvisable 
to do so. This is not merely because of the deluge of names that would result, 
but especially because of intimate and often inextricable relations of these 
four elements. 

Correlation of divergent views. — The extreme range of opinion as to the 
concept of the formation is afforded by the views of Gradmann and of 
Warming. The one would "ground formations solely upon floristic," the 
other expressly states that the "formation is not concerned with floristic." 
Both clearly demonstrate that a partial view is unfortunate, and serve to 
convince the open-minded student that only the complete point of view, which 
includes all of the relations of habitat and formation, is scientifically tenable. 
Every investigator has been concerned primarily with one relation and has 
minimized or neglected all the others. As a consequence, every standpoint 
has had its vigorous advocates, with the result that their arguments have 
proven each other partly right and partly wrong. It is clear why physiog- 
nomy as the most obvious basis should have first dominated the concept, and 
why it should have been displaced more and more by floristic. In both cases 
the habitat could not well be completely ignored, but its real value could be 
appreciated only after it began to be studied by means of instruments. Devel- 
opment is the most recent phase of formational study, and has in consequence 
played little part in determining the concept. The recognition of its funda- 
mental role in no wise minimizes the importance of the other viewpoints, 
since, it is an epitome of them all. It is also true that habitat, floristic. and 
physiognomy are complementary and not antagonistic. A complete picture 
of the formation is impossible without all of them, and the question of relative 
importance, if of any consequence at all, is a matter for much more detailed 
and thorough investigation than we have had up to the present. 



124 STRUCTURE AND UNITS OF VEGETATION 

Hence there is here no intention of setting up another antagonistic concept 
of the formation, i. e., one based upon development. The actual recognition 
of formations by means of physiognomy, of floristic, and of habitat has been 
tried repeatedly by the use of detailed and exact methods of quadrat and 
instruments. This has afforded conclusive proof that no one of the three 
viewpoints is adequate alone or primarily. This conclusion is reinforced by 
the conflicting opinions of the advocates of the different concepts, but espe- 
cially by the intensive study of the interrelations of community and habitat. 
Every community not only owes its grouping or composition to the habitat, 
but the species, and especially the dominant ones, take their characteristic 
impress from it. While their reproduction-form or taxonomic form shows 
this least for obvious reasons, the vegetation-form, growth-form, or life-form 
usually affords a striking illustration of this fact, and the habitat-form is an 
exact and universal record of it. On the other hand, the community modifies 
the habitat materially or essentially by its reactions upon it, and the habitat 
thus changed has a new action in selecting and modifying the species which 
enter it. This maze of action and reaction continues from the beginning to 
the end of the life-history of the formation, and it is as one-sided and unfor- 
tunate to emphasize one precess as it is the other. The habitat is the basic 
cause, and the community, with its species or floristic, and its phyads and 
ecads, or physiognomy, the effect. But the effect in its turn modifies the 
cause, which then produces new effects, and so on until the climax formation 
is reached. A study of the whole process is indispensable to a complete under- 
standing of formations. One must perforce conclude that the results obtained 
from the over-emphasis of physiognomy, floristic, or habitat are as incomplete 
as the concept itself. The simultaneous study of all the processes and facts 
can not yield too much truth, and it is a distinct handicap to assume that a 
single viewpoint can afford all or most of the truth. 

Significance of development. — It is for these reasons that development is 
taken as the basis for the analysis of vegetation. It is not a single process, 
but a composite of all the relations of community and habitat. It not only 
includes physiognomy, floristic and habitat, but it also and necessarily in- 
cludes them in just the degree to which they play a part, whatever that may 
be. Development furnishes, not a new point of view more or less incomplete 
and antagonistic to those already existing, but one which includes all the 
others and harmonizes and definitizes them. Its importance is just as great 
and its use just as fundamental as in taxonomy. The artificial system of 
Linnjeus was not unnatural because it failed to use natural characters, but 
because it used only part of them, and these not in their most fundamental 
relations. So, likewise, all the concepts of the formation and the methods of 
recognition so far employed are natural in so far as they use a natural process 
or response, and artificial in so far as they fail to correlate this with all the 
other equally natural and important processes. Taxonomic systems have 
become natural and hence fundamental in just the proportion that it has been 
possible to ground them upon development. Development is likewise the only 
basis for a natural system of formations. It is as indispensable to their 
recognition as to their classification. 

Earlier suggestions of the developmental view. — The fact that develop- 
ment has more than once been used in classifying communities indicates that 



THE FORMATION 125 

the idea has not been wholly ignored in the formational concept. All of tL? 
writers upon retrogression and regeneration of communities have had an 
inkling of this fact, but have nowhere expressed it in the formational concept. 
Drude suggests the idea more or less incidentally in his definitions (1890: 29; 
1896:286) when he speaks of a formation reaching an end in itself. Pound 
and Clements (1898:216; 1900:315) distinguished formations as either primi- 
tive or recent with respect to origin, and stated that formations originate at 
the present day by one of two principal methods, by nascence or by modifica- 
tion. Schimper's (1898) much-discussed division of formations into climatic 
and edaphic was really based upon development, but he failed to recognize the 
fundamental and universal nature of edaphic formations as processes of 
development. In his physiographic ecology, Cowles (1901) dealt primarily 
with development, though this fact was obscured by the emphasis laid upon 
physiography. However, he used the term "society" in place of "forma- 
tion, ' ' and his developmental ideas were not embodied in the formational con- 
cept. Clements (1902; 1904:6; 1905:199; 1907:219) advanced the concept 
that the formation was essentially developmental in character, and stated 
that it may be regarded as a complex organism which shows both functions 
and structure, and passes through a cycle of development similar to that of 
the plant. Transeau (1905:886) also adopted a similar view in the statement 
that ' ' each formation is made up of many societies, bearing a definite succes- 
sional relation to one another." He made no concrete applications of his 
view, and hence it remains ambiguous. Moss (1907:12; 1910:36) proposed 
a view similar to the two preceding, in which, however, the limitation of the 
formation was grounded primarily upon the habitat. Tansley (1911:9) has 
adopted Moss's concept, and defines the formation as follows: 

"In the normal primary development of a formation, the associations 
involved show intimate relations and transitions one to another, and the 
whole set of associations has a definite flora dependent on the type of soil. 
It is for these reasons that we consider the entire set of plant communities 
on a given type of soil, in the same geographical region, and under given 
climatic conditions, as belonging to one formation, in spite of the diversity of 
the plant forms in the different associations. The plant formation thus 
appears as the whole of the natural and semi-natural plant-covering occupy- 
ing a certain type of soil, characterized by definite plant communities and a 
definite flora." 

As has elsewhere been shown, the developmental value of this concept has 
been greatly reduced by linking the habitat to a type of soil. 

THE FORMATION. 

Developmental concept of the formation. — In spite of the growing tend- 
ency just indicated, no attempt has hitherto been made to put the formation 
either chiefly or wholly upon a developmental basis. While this view has 
been stated and restated in the preceding pages, it seems desirable to repeat 
it here at some length. The unit of vegetation, the climax formation, is an 
organic entity. As a complex organism, the formation arises, grows, matures, 
and dies. Its response to the habitat is shown in processes or functions and in 
structures which are the record as well as the result of these functions. 
Furthermore, each climax formation is able to reproduce itself, repeating 



126 STRUCTURE AND UNITS OF VEGETATION 

with essential fidelity the stages of its development. The life-history of a 
formation is a complex but definite process, comparable in its chief features 
with the life-history of an individual plant. The climax formation is the 
adult organism, the fully developed community, of which all initial and 
medial stages are but stages of development. Succession is the process of the 
reproduction of a formation, and this reproductive process can no more fail 
to terminate in the adult form in vegetation than it can in the case of the 
individual plant. 

The underlying causes of complete development of the formation are to be 
sought in the habitat, just as they are in the case of the individual. The sig- 
nificant difference lies in the fact that the reactions of the individuals as a 
community produce a cumulative amelioration of the habitat, a progressive 
improvement of the extreme, intrinsic to the continuance of development 
itself. The climax formation is thus a product of reaction operating within 
the limits of the climatic factors of the region concerned. A formation, in 
short, is the final stage of vegetational development in a climatic unit. It is 
the climax community of a succession which terminates in the highest life- 
form possible in the climate concerned. 

Analysis of the formation. — Just as development determines the unit of 
vegetation to be the climax formation, so it also furnishes the basis for recog- 
nizing the divisions into which the formation falls. It is evident that the final 
stage of a sere differs from all the preceding ones in a number of respects, but 
chiefly in being fixed throughout a climatic era. It is in essential harmony 
with its habitat, and no change is possible without a disturbance from the 
outside. Its own reaction is neither antagonistic to itself nor more favorable 
to other species. In the case of all the other successional stages, their respec- 
tive communities persist for a time only because their lack of harmony with 
the climatic conditions is counterbalanced by a more or less extreme set of 
edaphic conditions. Sooner or later this compensating relation is destroyed 
by the progress of the reaction, and the one stage is replaced by another. As 
a consequence, the formation falls naturally into climax units or associations, 
and developmental or serai units, associes. The former have their limits in 
space, and are permanent for each climatic era ; the latter are limited in time, 
and they arise and pass in the course of successional development. Serai 
units represent the visible or determinable stages of development, and hence 
include all the successive communities of a sere. Each associes is based in 
consequence upon population, life-form, and habitat, though it is most readily 
distinguished by means of its dominant species. It is not certain that the 
major changes in dominance and life-form coincide with the major changes of 
the habitat, but quantitative studies point more and more to this conclusion. 

Formation units. — Moss (1910:20, 27) has traced in detail the develop- 
ment of the concepts of formation and association, as well as their varying 
use, while Plahault and Schroter (1910) have made an illuminating summary 
of them in connection with phytogeographic nomenclature. The first en- 
deavor to analyze these units more minutely was made by Clements ( 1905 : 
296, 299), who proposed society, community, and family as respective subdi- 
visions of the association. A similar division of the formation into types, 
facies, aspects, and patches had been made by Pound and Clements (1898: 
214; 1900: 319) and Clements (1902:19), but the essential nature of the type 



THE FORMATION 127 

as a subdivision of the formation was obscured by a double use of the latter 
term. The term society was adopted by Moss (1910) and Tansley (1911), 
and has been used more or less generally by British ecologists. The latter 
have also tended to employ community as an inclusive term for any and all 
units from the formation to the family. Convenience and accuracy demand 
such a term, and it is here proposed to restrict community to this sense. For 
its concrete use to designate the division next below the society, the term 
clan is proposed. 

The term consocies was first proposed (Clements, 1905:296) as a substitute 
for association, owing to the use of the latter in both an abstract and a con- 
crete sense. The general use of association in the concrete sense has fixed it 
definitely in ecological terminology. At the same time, its actual application 
to particular communities has shown the widest divergence of viewpoint. As 
a consequence of more exact knowledge of vegetation, it became evident that 
a new division was needed between association and society to designate the 
characteristic dominance of faeies (Pound and Clements, 1900:319). The 
term consocies has been used for this division, since this is precisely the unit 
for which it was first proposed. Thus, while the relation of formation and 
association remains the same, consocies would become the term to be applied 
to by far the larger number of associations as hitherto recognized. This con- 
cept in particular has been repeatedly tested during the past two years 
throughout the western half of North America, and has shown itself to be one 
of the most valid and easily applied of all the units. The term has been used 
in this sense or essentially so by Shantz (1906:36), Jennings (1908:292; 
1909:308), Gleason (1910:38), Gates (1912:263), Matthews (1914:139), and 
Vestal (1914:356; 1914:383) (plate 35 opposite). 

However, the requirements of a developmental analysis of vegetation make 
it desirable, if mot necessary, to distinguish between climax and develop- 
mental consocies. Accordingly, it is proposed to retain consocies for the serai 
unit, and to employ consociation for the climax unit. Thus, from the stand; 
point of structure, the following plant communities are recognized, namely, 
formation, association, consociation, society, clan, and family. Their essen- 
tial relationship is indicated by the sequence. Since at least the formation, 
consociation, and family permit of objective limitation, the use of the remain- 
ing terms may be definitized much more than has been the ease hitherto. 

Formation. — The formation is the unit of vegetation. It is the climax com- 
munity of a natural area in which the essential climatic relations are similar 
or identical. It is delimited chiefly by development, but this can be traced 
and analyzed only by means of physiognomy, floristic, and habitat. In a 
natural formation, development, physiognomy, and floristic are readily seen 
to be in accord, but this often appears not to be true of habitat. There are 
several reasons for this. In the first place, complete and exact knowledge of 
any habitat is still to be obtained. As a consequence, the actual correlation 
of factors and the critical responses of the plant are as yet untouched. Fin- 
ally, we think of climate in human terms, and forget that the only trust- 
worthy evidence as to climatic climaxes must be obtained from the responses 
of the plant and the community. Even the exact evidence obtained by record- 
ing instruments may be most misleading, unless it is translated into terms of 
plant life. Thus, while there is every certainty theoretically that the respon- 



128 STRUCTURE AND UNITS OF VEGETATION 

sive unit, the formation, is in harmony with the causal unit, the habitat, our 
present knowledge is inadequate to prove this. As a consequence, the habitat 
can only be used in a general way for recognizing formations, until we have a 
much clearer understanding of the climatic and edaphie factors and the 
essential balance between them. 

The developmental limitation of formations demands long investigation. 
Hence it is necessary to appeal first to physiognomy and floristic for tentative 
units, except in regions where successional studies are already well advanced. 
Such tentative units must be tested and confirmed by development before 
they can be accepted. Such a test will necessarily involve the use of habitat 
criteria to an increasing degree. Thus, over the whole of the Great Plains 
region, life-forms and population indicate a vast grassland formation. The 
existence of such a climax is confirmed by numerous developmental studies 
which have already been made upon it. In the matter of temperature the 
region is far from uniform, but in the critical water relations investigation 
shows it to be essentially a unit. Over this wide stretch from Texas and 
northern Mexico far into Alberta, the dominant genera are the same, and this 
is true of many of the species. This is also true of the genera and some of the 
species of the scrub or chaparral formation which extends from Minnesota 
westward to British Columbia, southward to California and Mexico and east- 
ward to Texas, Colorado, and Nebraska. 

According to the developmental idea, the formation is necessarily an organic 
entity, covering a definite area marked by a climatic climax. It consists of 
associations, but these are actual parts of the area with distinct spatial rela- 
tions. The climax formation is not an abstraction, bearing the same relation 
to its component associations that a genus does to its species. It is not a 
pigeon-hole in which are filed physiognomic associations gathered from all 
quarters of the earth. Hence it differs radically from the formation of "Warm- 
ing and other writers who have adopted his concept. According to the latter 
(1909:140; cf. Moss, 1910:43), "a formation appears with a certain deter- 
mined uniformity and physiognomy, even in different parts of the world, and 
even when the constituent species are very different and possibly belong to 
different genera and families. Therefore a formation is an expression of cer- 
tain defined conditions of life, and is not concerned with floristic differences. ' ' 
The formation as developmentally limited would include the closely related 
chief associations of Drude (1896:286) and Moss (1910:38). The formations 
of many writers are associations as here understood, and those of Hult and 
his followers are mostly consocies and societies. The current conceptions of 
formation and association in the larger sense were regarded as fairly final by 
the writer, until 15 months of continuous field-work in 1913 and 1914 made 
this position appear to be no longer tenable. This change of view was not 
only a direct consequence of the application of developmental principles to a 
wide range of communities, but it was also rendered unavoidable by the 
opportunity of comparing all the formations and associations in the region 
from the prairies to the Pacific Coast, and between Mexico and middle 
Canada during the summer of 1914. 

Names of formations. — The need of being able to designate formations 
more accurately than by the use of vernacular names led to the proposal by 
Clements (1902:5) that they be designated by Greek names of habitats or 



CLIMAX UNITS 129 

communities, to which the suffix, *uov, place, was added. This suggestion 
has been adopted by Ganong (1902:53; 1903:303), Diels (1908:70; 1910:18), 
and Moss (1910:142; 1913:167). More recently, Brockmann and Riibel 
(1912), and Riibel (1915) have proposed a physiognomic system based upon 
Latin. The physiognomic basis seems much less satisfactory, and the use of 
Latin compounds certainly leaves much to be desired in the matter of uni- 
formity, brevity, and euphony. While Clements and Diels use the trans- 
literated form of the suffix, as in hylium, helium, etc., Moss objects to this 
because of the fear that it would lead to confusion with neuter generic and 
specific names. Such confusion would be impossible if the formational terms 
are not capitalized, as was originally intended. Since uniformity is more 
desirable than any other feature of terminology, the modification of the term 
by Moss is accepted here, as it has become more or less current in British 
publications. Since climax formations are clearly dependent upon the flora, 
it seems impossible to ignore this fact in the name. Moss objects to the use 
of the names of dominant genera, as in " Eriophorum-Scirpus oxodion, " 
because it is not really definitive, as no indication is given of the species of 
Eriophorum or Scirpus. Further objection is raised because the oxodion com- 
prises not merely the two associations designated, but probably at least two 
dozen. These objections disappear in the developmental treatment of forma- 
tions, since there are rarely more than a few associations in a formation. If 
Bulbilis-Bouteloua-poion is thought too long for the name of the short-grass 
climax of the Great Plains, it can be called simply Bouteloua-poion, just as a 
similar climax elsewhere might be the Stipa-poion. The greater definiteness 
both as to floristic and region seems to render such formational names prefer- 
able to Moss's a-oxodion, fi-oxodion, etc. 

CLIMAX UNITS. 

Association. — The association has had as varied a history as the forma- 
tion. Not only has the one been used for the other, but even when they have 
been employed in the proper relation the units to which they have been 
applied have varied greatly. As has been already indicated, the association 
as usually understood becomes what is here termed the consociation, in so far 
as it is a climax community. This is the association with a single dominant. 
While many associations of two or more dominants have been recognized, 
these are practically all what Moss (1910:38) terms subordinate associations, 
that is, successional communities or associes (plate 15, a, b). 

The association as here conceived bears the accepted relation to the forma- 
tion. The term is restricted, however, to those climax communities which are 
associated regionally to constitute the formation. The associations agree with 
their formation in physiognomy and development, but differ in floristic and 
to a certain though unknown degree in habitat. Hence they are recognized 
chiefly by floristic differences. Associations are marked primarily by differ- 
ences of species, less often by differences of genera. At the same time, their 
organic relation to each other in the climax unit or formation rests upon 
floristic identity to the extent of one or more dominants, as well as upon 
the fundamental development and the life-forms. For example, the 
Bouteloua-poion contains two associations, the B nihil is-Bouteloua-associati&n, 

10 



130 STRUCTURE AND UNITS OF VEGETATION 

and the Aristida-Bouteloua-association. While the species of Bouteloua and 
Aristida are mostly different in the two, one or more species of both genera 
are more or less common throughout. In the scrub or chaparral formation, 
Quercus, Ceanothus, Cercocarpus, and Rhus are common genera, with one or 
more common species. Associations show a similar relationship with refer- 
ence to the principal and secondary species. The great majority of these are 
the same as to genera, and the number of identical species is usually consid- 
erable. 

From the organic connection between formation and association, it seems 
desirable to use similar terms to designate them. For the sake of distinction, 
however, it is necessary to employ the termination in different form. Accord- 
ingly, it is proposed to use the roots found in hylion, helion, poion, etc., but 
to substitute the ending -ium for -ion: Thus, the short-grass formation, 
Bouteloua-poion, of the Great Plains would fall into the Bulbilis-Bouteloua- 
poium and the Aristida-Bouteloua-poium. This method has the advantage of 
definitely correlating formation and association upon the basis of life-form 
and habitat, and of reducing the number of terms needed. The names thus 
constituted are so few and so distinctive that there seems not the slightest 
danger of confusion with neuter generic names (cf. p. 182). 

Consociation. — The consociation is the unit of the association. It is char- 
acterized by a single dominant. The association is actually a grouping, the 
consociation is pure dominance. Hence it is the most readily recognized of all 
communities, and it has figured both as formation and association. In the 
usual treatment most consociations appear as associations. This fundamental 
relation between formation, association and consociation was recognized by 
Pound and Clements (1898:223, 1900:324) in the division of the river- 
bluff formation into the red oak-hickory type, and the bur oak-elm-walnut 
type, each characterized by a number of dominant species or facies. While 
the communities are now seen to have been too restricted, the sequence of 
formation, type, and facies is essentially that of formation, association and 
consociation. A similar relation between the facies and consocies was recog- 
nized by Clements (1907:226). As a consequence, it is but a short step to 
clarify this relation into the exact one here established between association 
and consociation. The association thus becomes a group of two or more con- 
sociations, and the word "facies" disappears in this sense at least. 

The uniform dominance of a consociation makes its recognition a simple 
matter. Since the consociations of an association approach each other in 
equivalence, i. e., in response to the habitat, they are frequently mixed in 
various degrees. Such mixtures are more or less complete expressions of 
the association, however, and are so numerous and various that no definite 
term is required. The Bulbilis-Bouteloua-poium consists of two divisions, the 
Bouteloua consociation and the Bulbilis consociation; the Aristida-Bouteloua- 
poium of several consociations, Bouteloua rothrockii, B. eriopoda, Aristida, 
arizonica, etc. When two or more consociations are mixed, the term mictium 
(Clements, 1905:304) may be employed when needed, as for example, a 
Bubilis-Bouteloua-mictium would be an area of mixed grama and buffalo- 
grass, which, with the Bouteloua and Bulbilis consociations, would make up 
the association. Such a mictium is, however, only the association in minia- 
ture. 



CLEMENTS 




A. Montane forest association, Pinux-Abits-ln/liiim (l\ ponderosa, 

P. lambertiana, A. concolor), Yosomite, California. 
B. yellow-pine consociation, Pinetum ponderosae, Prospect, Oregon. 



CLIMAX UNITS 131 

A consociation is denoted by the term -etum, a suffix long ago proposed by 
Schouw (1823:165) for a community characterized by a single dominant. 
This termination has come into general use, usually for a single dominant, 
though frequently for a group of related or associated dominants. It is here 
restricted to the climax community formed of a single dominant, i. e., the 
consociation, for example, Boutelouetum, Bulbiletum, Aristidetum, Querce- 
tum, Bhoetum, etc. 

Society. — The society is a community characterized by a subdominant or 
sometimes by two or more subdominants. By a subdominant is understood 
a species which is dominant over portions of an area already marked by the 
dominance of consociation or association. The society is a localized or 
recurrent dominance within a dominance. In the case of grassland, tUe 
striking subdominance of many societies often completely hides the real 
dominance of the consociation. In forest, societies are found only beneath 
the primary layer of trees, and their subdominance is obvious. The society 
comes next below the consociation in rank, but it is not necessarily a division 
of it, for the same society may extend through or recur in two or more con- 
sociations, i. e., throughout the entire association. This seems readily under- 
standable when we recognize that the life-forms of the society subdominants 
are regularly different from those of the dominants of grassland and forest. 
The societies of grassland are composed of herbs or of undershrubs rather 
than grasses, those of woodland, of herbs, bushes, and shrubs. They may 
occur more or less uniformly over wide stretches, or they may be repeated 
wherever conditions warrant (plate 16 a). 

The concept of the society was proposed by Clements (1905:296) and was 
denned as follows : 

"The seasonal changes of a formation, which are called aspects, are indi- 
cated by changes in composition or structure, which ordinarily correspond to 
the three seasons, spring, summer, and autumn. The latter affect the facies 
[consociations] relatively little, especially those of woody vegetation, but they 
influence the principal species profoundly, causing a grouping typical of each 
aspect. For these areas controlled by principal species, but changing from 
aspect to aspect, the term society is proposed. They are prominent features 
of the majority of herbaceous formations, where they are often more striking 
than the facies. In forests they occur in the shrubby and herbaceous layers, 
and are consequently much less conspicuous than the facies." 

Later (1907:226), the concept was somewhat broadened: 

"An area characterized by a principal [subdominant] species is a society. 
A society, moreover, is often characterized by two or more principal species. 
Societies have no essential connection with consocies. A eonsocies may include 
several or many societies, or it may not show a single one. Finally, a society 
may lie in two consocies, or it may occur in any of them." 

Tansley (1911:12) and his co-workers have adopted the concept of the 
society, and have stated it as follows: 

"Locally within an association there occur more or less definite aggrega- 
tions of characteristic species or of small groups of species, and these, which 
appear as features within the association, may be recognized as smaller vege- 
tation units, or plant-societies. Sometimes their occurrence may be due to 
local variations of the habitat, at other times to accident and the gregarious 



132 STRUCTURE AND UNITS OF VEGETATION 

habit originating from a general scattering of seed in one place, or from the 
social growth of a rhizomic plant. It is a question whether it would not be 
better to separate these two causes of the production of societies within an 
association, and to restrict the term society to aggregations due to the latter 
alone. In this way we should obtain a more logically coherent conception. 
But the more detailed analysis of vegetation has hardly progressed far 
enough at present to justify a finer classification of plant communities. While 
a plant formation is always made up of associations, an association is not 
always or even necessarily made up of societies, which are essentially local 
discontinuous phenomena. Finally, plant-societies are minor features of vege- 
tation, and their presence in certain spots is generally determined by some 
biological peculiarity, not by the habitat as such." 

Moss (1910 :48) states that "it is becoming usual in this country to speak of 
the subdivisions of the association as plant societies (c/. Clements, 1905 :296)," 
and (1913:19) that "a plant society is of lower rank than an associa- 
tion, and is marked by still less fundamental differences of the habitat." 
The facies and " nebenbestande" of many authors are societies, as are also 
many of the patches of Pound and Clements (1898:214; 1900:313) and 
Clements (1902:19). The concept of the society has further been adopted 
and applied by Shantz (1906:29; 1911:20), Young (1907:329), Jennings 
(1908:292; 1909:308), Ramaley (1910:223), Adamson (1912:352), and 
Vestal (1914 2 :383). 

Bases. — "While the concept of society arose from the dominance of prin- 
cipal species, and thus has always had more or less relation to seasonal aspects, 
there is no necessary connection between the two. In the prairie association 
the seasonal appearance of societies is a marked phenomenon. In other com- 
munities the four aspects, prevernal, vernal, festival, and autumnal, may be 
reduced to two or even one, and a society may then persist through much or 
all of the growing season. Even when the aspects are well-marked, a particu- 
lar society may persist through two or more. As a consequence, the question 
of time relations is not a necessary part of the concept, though it may prove 
desirable to distinguish societies with marked seasonal character. 

The real warrant for the recognition of societies lies in the structure, and 
hence in the development of the formation also. Areas of characteristic 
dominance occur within the major dominance of consociation and association. 
Such communities can not be ignored, for they are just as truly a part of the 
plexus of habitat and vegetation as the consociation itself. They are an essen- 
tial result of the interaction of physical and biological processes, and the 
explanation of their occurrence is necessarily to be sought in the habitat. As 
Tansley has suggested (1911:12), it may prove desirable to distinguish socie- 
ties controlled by obvious differences of habitat from those in which such con- 
trol is lacking or obseure. This seeems a task for the future, however, since it 
depends primarily upon the instrumental study of units, of which we have the 
barest beginning. Moreover, it appears evident that the vast majority of 
societies, if not all of them, are expressions of basic habitat relations. This 
must certainly be true of the societies of climax associations and consociations, 
and it must also be the general rule in the case of the developmental societies 
of a sere. The only obvious exceptions are furnished by ruderal or subruderal 
species which invade quickly and remain dominant for only a few years. In 



CLIMAX UNITS 133 

the Great Plains the societies of Eriogonum, Psoralea, Helianthus, etc., which 
occur and recur over thousands of square miles, have had abundant time and 
opportunity to migrate over the whole region ; Psoralea tenuijlora is found, in 
fact, from Illinois and Minnesota to Texas, Sonora, Arizona, and Montana. 
Hence the presence of the society over large stretches and its absence in other 
places must be a matter of habitat control. In this, naturally, competition 
must often play a dominant part, and there can be little question that exact 
analysis will some day enable us to distinguish some societies upon this basis 
(Woodhead, 1906:396; Sherff, 1912:415). At present, such a distinction is 
impossible or at least without real meaning. Hence, while societies are readily 
seen to range from complete dominance, often greater than that of the con- 
sociation, to mere characteristic, it is highly probable that these merely repre- 
sent different degrees of habitat response. This is often not obvious, for the 
decisive effect of the factors which control a society may be felt only at the 
time of germination for example, and might easily escape one who failed to 
use the exact methods of quadrat study throughout the entire growing-season. 
Perhaps no better evidence of the relation of societies to habitat can be fur- 
nished at present than their striking variation in abundance from one area to 
another, when such areas show no visible habitat differences. As a conse- 
quence, while it is possible to regard some societies as dominant, and others as 
only characteristic, it is felt that such a distinction is merely one of degree. 
It is necessarily superficial in the present state of our knowledge, and has the 
further disadvantage of being too easily subjective. An experimental study 
of dominance might well furnish a real basis for distinctions here, but further 
analysis must await such study. 

Kinds of societies. — There may well be differences of opinion as to the 
desirability or necessity of distinguishing various types of societies. Those 
who are more interested in other phases of vegetation than in its development 
and structure will naturally not need to use finer distinctions. On the con- 
trary, those who wish to traee in detail the response of the community to its 
habitat will find it helpful to recognize several kinds of societies. Even here, 
however, it is undesirable to outrun our present needs and to base distinctions 
upon differences which are subordinate or local. Thus, while it is convenient 
and natural to recognize layer societies, it would result in a surplus of con- 
cepts and terms to distinguish societies upon the basis of the six or eight 
layers present in well-lighted forests. Accordingly it seems desirable to regard 
all societies as due to habitat control, more or less modified by competition, 
and to establish subdivisions only upon the following bases: (1) aspects, 
(2) layers, (3) cryptogams. In addition, there are the relict and nascent 
societies of various serai stages, which will be considered under develop- 
mental societies. Finally, there are the related questions of changes of rank 
or dominance, which are dealt with below. 

Aspect societies. — Since most societies are composed of subdominant herbs, 
x. e., dominant within a dominance, their chief value usually appears as they 
approach maturity, and especially when they are in flower. Astragalus crassi- 
carpus, for example, is present in the prairie from early spring to frost. But 
it dominates hillsides only in the spring, before the taller herbs have grown, 
and this dominance is a conspicuous feature only when the plants are in 
bloom. There is, then, a seasonal change of dominance which marks the 



134 STRUCTURE AND UNITS OF VEGETATION 

aspects of vegetation. In open woods a similar change of dominance results 
from the successive appearance of the layers, the earlier lower layers being 
masked by taller later ones. Thus there may be distinguished prevernal, 
vernal, festival, and autumnal aspects, and corresponding societies. In boreal 
and alpine regions the number of aspects is often but two, vernal and sestival, 
and the societies correspond. The large majority of societies fall more or less 
clearly within one aspect, but there are exceptions, as previously suggested. 
Hence it is necessary to establish a major distinction into aspect societies and 
permanent societies. Many of the latter are not true societies at all, but are 
more or less imperfect expressions of undershrub and scrub consocies which 
represent a potential climax. Such are the Gutierrezia, Yucca, and Artemisia 
cana communities of the Great Plains. 

Layer societies. — As already indicated, these usually have a seasonal rela- 
tion also, as they tend to develop successively rather than simultaneously. The 
societies of thicket and woodland differ from those of grassland in being more 
coherent and in falling into well-marked layers. The latter are found in 
prairie, but they are usually incomplete and obscure. When the development 
of the layers is clearly seasonal, the societies concerned may well be regarded 
as aspect societies. As a rule, however, the layers are all developed before 
midsummer, and the forest presents a distinctively storied appearance. Nat- 
urally, the layers are often fragmentary or poorly defined, and in closed or 
mature forests they may be lacking. It seems best, then, to distinguish but 
two kinds of layer societies at present, namely, societies of the shrub layer or 
layers, and societies of the herbaceous layers. In cases where tall herb layers 
overtop one or more of the shrub layers this distinction has little value, but 
as a rule, the essential difference in the life-forms of the two layers or sets of 
layers marks a convenient if not an important distinction (c/. Hult, 1881). 

Cryptogamic societies. — These in turn bear some relation to layer and sea- 
sonal societies. The lowermost layer of a thicket or forest often consists of 
mosses, liverworts, lichens, and other fungi. In mature forests of spruce this 
is often the sole layer. Nearly all the parasites and many of the saprophytes 
can not develop until stems and leaves appear, and hence exhibit both a sea- 
sonal and a layer relation. While there can be no question of the distinctness 
of cryptogamic societies, their treatment is a difficult matter. Many of them 
are actual colonies in minute seres, such as the pure or mixed communities of 
Marchantia, Funaria, or Bryum in burned spots. Distinctions into ground 
societies, parasitic societies (i. e., those mostly on leaves and herbaceous stems, 
which necessarily disappear each season), and bark societies (which persist 
from one year to another) are convenient, but of minor importance. A dis- 
tinction based upon life-form, i. e., moss, liverwort, lichen, and fungus, is 
probably of greater value. Perhaps a more exact analysis would result from 
the use of both life-form and location, but such a basis produces results too 
detailed for our present needs. The soil in particular presents a virgin field 
for the recognition and limitation of parasitic and saprophytic societies and 
socies, especially of bacteria, but our knowledge is too slight to furnish the 
necessary criteria. 

Terminology.— Societies have been designated by adding the locative 
suffix -He to the name of the dominant genus, e. g., Iridile, Opulasterile,< 
Androsacile (Clements, 1905:299). Layers were named in similar fashion by 



CLEMENTS 




A. Lupine society, Lupinili plattensis, in mixed prairie, Hat Crock Basin, Nebraski 
B, Clan of Pirola elliptica in forest, Lake Calhoun, Minnesota. 



CLIMAX UNITS 135 

adding -anum to the generic name or group, Opulaster-Ribesanum. Since the 
society is usually a group with a definite impress and a basic relation to 
habitat conditions, much as in the consociation, it seems appropriate that it 
should likewise bear a locative suffix. For these reasons the suffix -He is 
retained to designate societies in general and aspect societies in particular. 
It may well serve also for thallophytic societies, e. g., Funarile, Marchanlile, 
Cladonile, Agaricile, since the generic name clearly indicates the life-form. 
When it seems desirable to distinguish layer societies, it may likewise be done 
most simply and briefly by means of a suffix. Since the suffix -anum, already 
used for layer, is unnecessarily long, it is proposed to replace it by -en, e. g., 
Fragarien, Thalictren, Erythronien, Helianthen. Where mixed societies exist 
there is no better method than to combine the two generic names, e. g., 
Psoralea-Helianthile. 

Changes of rank or dominance. — Since consociation and society are based 
chiefly upon dominance as controlled by habitat, it sometimes happens that 
the dominance changes to such a degree or over such an area that the com- 
munity loses its usual rank. A consociation may appear to be a society, or 
even a clan. A society may assume the appearance of a consociation, or, 
on the other hand, may likewise be so reduced as to resemble a clan. Such 
changes in value occur most frequently (1) in or near transition areas, (2) as 
a result of temporary oscillations of climate, and (3) in the course of succes- 
sional development, during which consociations may dwindle to insignificant 
groups, or colonies which appear to be societies or clans develop into consocia- 
tions. The last is typically the case with such undershrubs as Gutierrezia, 
Artemisia, Yucca, etc., which often appear as clans and societies in grassland. 
These communities, however, are really the beginnings of a postclimax conso- 
ciation, the full development of which is conditioned upon a climatic change. 

It is possible to treat communities of this sort solely with reference to their 
actual value in a particular association, without regard to their normal or 
developmental relation. Such a method is simpler and more convenient, 
but it has the disadvantage of obscuring the organic relations and of confusing 
the facts of development. Consequently it is thought best to regard consocia- 
tions and societies as entities, which may increase or decrease markedly in 
dominance and extent under certain conditions. However, if the facts are 
made clear, it matters little whether a particular group is called a reduced 
consociation or a society which represents a consociation of a contiguous area. 
Theoretically it is possible, at any rate, that a consociation of one region may 
be changed into a typical society in another. 

Clan. — A clan is composed of a secondary species. It is next below the 
society in rank, though it is not necessarily a subdivision of it. Clans may 
and usually do occur in societies, but they are also found in consociations 
where there are no societies. A clan differs from a society chiefly in being 
local or restricted to a few small and scattered areas. Its dominance is slight 
or lacking, though it may often furnish a striking community in the vegeta- 
tion. While societies and clans can usually be distinguished with readiness, 
there is no hard and fast line between them. Even the use of quadrat 
methods can not always distinguish them clearly. A elan differs from a colony 
in being a more or less permanent feature of climax communities or of con- 
socies which exist for a long time. A colony is a group of two or more 



136 STRUCTURE AND UNITS OF VEGETATION 

species which, develops in a bare area or in a community as an immediate 
consequence of invasion (plate 16 b). 

Clans are distinguished upon the same bases as societies. They are con- 
nected for the most part with particular aspects, and the vast majority of 
them are aspect clans. The minor groups of layers are layer clans, and the 
clan may also be recognized in the moss, lichen, and fungus communities. The 
term clan is a partial synonym for community in the original sense (Clements, 
1905: 297; 1907:227, 240). It comprises the communities found in subclimax 
and climax stages, while the invading or developing communities of initial 
groups are termed colonies. Communities have been designated by means of 
the suffix -are (I. c, 1905 :299), and it is now proposed to restrict the use of 
this suffix to the clan, e. g., Gentianare, Mertensiare, etc. 

SERAL UNITS. 

Nature and significance. — The units which have just been considered are 
essentially climax communities. In addition, there are similar or analogous 
communities throughout the course of succession. To many it will appear 
an unnecessary if not an unwelcome refinement to recognize a developmental 
series of units. To such students the series already established, viz., forma- 
tion, association, consociation, society, and clan, will suffice for all units with- 
out regard to a distinction between developmental and climax phases of 
vegetation. However, for those ecologists who regard the formation as an 
complex organism, it is as essential to distinguish developmental and climax 
communities as to recognize gametophytic and sporophytic generations in the 
life-history of the individual. 

The need of such a distinction has already been at least suggested by Hult 
(1885) and Klinge (1892) in their recognition of climax formations, and 
especially by Drude (1890:29; 1896:286) when he states that he "regards as 
a vegetation formation each independent closed chief association of one or 
several life-forms, whose permanent composition is effected by the definite 
conditions of the habitat, which keep it distinct from the adjacent forma- 
tions." Schimper (1898) seems to have had some idea of this distinction in 
his recognition of climatic and edaphic formations, while Warming (1896: 
361; 1909:356) and Clements (1902:15; 1904:134) also suggested it in dis- 
tinguishing between initial, intermediate, and ultimate formations. Moss 
(1910:32), in this connection, says that: 

"As a definition of a closed, ultimate or chief association of a formation, 
this statement of Drude 's is excellent, though, as his 'formation' is essentially 
only a particular kind of association, it is not quite consistent with the views 
of those authors who regard the formation as related to the association as 
the genus is to the species. . . . From the point of view of succession, 
the formation of Drude, variously termed by him 'Formation,' 'Hauptforma- 
tion, ' and ' Hauptbestand, ' must be regarded as a chief association of a forma- 
tion. The chief associations of a district, however, do not comprise the whole 
of the vegetation of that district; they serve to give a vivid but somewhat 
impressionistic picture of such vegetation ; and the complete picture requires 
the addition of the details provided by the progressive and retrogressive asso- 
ciations, or, as these may be collectively termed, the subordinate associa- 
tions." (37) 



SERAL UNITS 137 

Moss (1910:36-38) further emphasizes the importance of distinguishing 
between climax and developmental associations: 

"A plant formation, then, comprises the progressive associations which 
culminate in one or more stable or chief associations, and the retrogressive 
associations which result from the decay of the chief associations, as long as 
these changes occur in the same habitat. . . . The above examples of 
succession are given in order to show the importance of regarding the forma- 
tion from the point of view of its developmental activities. . . . Every 
formation has at least one chief association ; it may have more ; and they may 
be regarded as equivalent to one another in their vegetational rank. They are 
more distinct and more fixed than progressive or retrogressive associations. 
They are usually, but not invariably, closed associations. They always repre- 
sent the highest limit that can be attained in the particular formation in 
which they occur, a limit determined by the general life conditions of the 
formation. ' ' 

Tansley (1911:12) has adopted the same view: 

"Thus each of the types of vegetation, woodland, scrub, and grassland, 
within a given formation, is a plant association, and so is each definite phase 
in the primary development of a formation. The highest type of association 
within a formation (often woodland), to which development tends, is called 
the chief association of the formation. In the absence of disturbing factors, 
such as the interference of man, land-slips, and so on, the chief associations 
will ultimately occupy the natural formation area to the exclusion of the 
other associations, which may collectively be termed subordinate associations." 

Cowles (1910) has also recognized the essential difference between develop- 
mental and final communities, in using the term ' ' climatic formation, ' ' which 
Moss (1910:38) points out is equivalent to his chief association. Moss regards 
Cowles 's term as unfortunate, because it is used in a very different sense from 
the same term of Schimper. This is hardly the case, for while Schimper's 
term covers more than one kind of unit, the recognition of climatic and 
edaphic formations seems clearly to have taken some account at least of devel- 
opment. (Cf. Skottsberg (1910:5) and Vestal (1914:383).) 

In spite of differences in their views of the formation, the nine authors just 
quoted, Hult, Klinge, Drude, Warming, Schimper, Clements, Moss, Cowles, 
and Tansley, have all distinguished more or less clearly between climax and 
developmental associations. Such a distinction naturally does not end with 
associations, but extends throughout the units. Hence it is here proposed to 
recognize and define a series of developmental units in the life-history of the 
climax formation, which is essentially analogous with association, consocia- 
tion, society, and clan. In fact, a failure to do this causes us to ignore prac- 
tically all the developmental study of the past 20 years, and to make the 
developmental analysis of vegetation difficult and confusing, if not impos- 
sible. 

Associes. — The associes is the developmental equivalent of the association. 
It corresponds to the initial and intermediate formations of Clements (1902, 
1904) and to the subordinate associations of Moss (1910) and Tansley (1911). 
It is composed of two or more consocies, i. e., developmental consociations, 
just as the association consists of two or more consociations. Like the asso- 
ciation, it is based upon life-form, floristic composition, and habitat, but differs 



138 STRUCTURE AND UNITS OF VEGETATION 

from it in as much as all of these are undergoing constant or recurrent devel- 
opmental changes. In so far as each sere is concerned, the associes is transient, 
though it may persist for many years, and the association is permanent. 
Obviously, a medial or final associes may become an association when the 
development is held indefinitely in a subclimax stage, as in heath and prairie. 
On the other hand, a change of climate which advances the climax converts 
the previous associations into developmental units, and they thus become 
associes. This potential relation between association and associes naturally 
obtains wherever climax associations are zoned. This is especially evident in 
mountain regions, where grassland and scrub associations are potential 
associes of the forest above (plate 17 a) . 

In its complete expression, the associes is marked by striking changes of 
both habitat and life-form, as necessarily of floristic. This is best illustrated 
in water seres, the initial stages of which constitute three well-marked associes, 
namely, submerged plants, floating plans, and swamp plants. In each there 
is a pronounced change of habitat necessarily accompanied by a correspond- 
ing change of life-form and floristic. While it seems probable that all impor- 
tant changes of life-form are due to the reaction upon the habitat, certainty 
in this respect is impossible as yet. It can be attained only by the instru- 
mental study of conditions before and after the change of life-form. Theo- 
retically, such a relation seems highly probable, and we may assume as a 
working hypothesis that one associes is delimited from another by important 
changes of habitat, as well as life-form. In the prisere of the spruce-fir 
formation, for example, it is probable that the change from crustose to foliose 
lichens is as great a change of habitat as happens anywhere in the sere, but 
it is too minute in extent to be impressive. 

While associes is obviously from the same root as association, it is based 
upon the original meaning of sequence (sec-, soc-, follow) rather than the 
derived one of companionship. Though the form assecies is preferable for 
some reasons, it is less euphonic and less suggestive of the relation of associa- 
tion. It seems desirable to emphasize the relationship between the two units 
by terms evidently related rather than to employ a wholly new word. For 
the same reason the names of particular associes are based upon the words 
already used for formation and association. These three units have so 
much in common that the same root modified by a different suffix seems to 
harmonize readily with the actual degree of similarity and difference. For 
associes, the termination -is is proposed, and we would thus have helis, 
pois, hylis, eremis, etc., e. g., Scvrpus-Typha-helis, Andropogon-Aristida- 
pois, etc. 

Consocies. — The consocies is a serai community marked by the striking or 
complete dominance of one species, belonging of course to the life-form 
typical of that stage of development. It is the unit of the associes in the 
same way that the consociation is of the association. The consocies and con- 
sociation differ only in that the former is a developmental or serai, the latter 
a climax, community. Bouteloua and Bidbilis are consociations of a climax 
association of the Great Plains, Andropogon scoparius and Aristida purpurea 
are consocies of a serai association, or associes. Because of its developmental 
nature, the reed-swamp association is an associes in the present conception, 
and each of its dominants, Scirpus, Typha, Phragmites, etc., forms a conso- 



CLEMENTS 



PLATE 17 







A. (!r;is.s associes of Andropogon and Calamovilfa, Crawford, Nebraska. 
B. Pentstemon socies (P. scat ml i /lor its) in gravel, Manitou, Colorado. 



SERAL UNITS 139 

cies. The heath association is likewise an associes, except where it may have 
been stabilized to form a climax, and Calluna and Erica form the character- 
istic consocies of it. 

The term consocies likewise is obviously related to consociation. In the 
latter, however, the suffix emphasizes the condition of being grouped together. 
In consocies, on the contrary, the emphasis lies upon the root seq- (sec-, soc-) 
found in sequor, and denoting sequence. This may be illustrated by the case 
of the reed-swamp consocies. As is well known, the three dominants are not 
exactly equivalent, but Scirpus usually invades the deepest water and Phrag- 
mites the shallowest, so that the corresponding consocies show a definite 
sequence, even though they are all present at the same time. Such a succes- 
sional relation is typical of the dominants of an associes, and it is just this 
relation which is denoted by the name consocies. It must also be recognized 
that an associes may be represented in one locality by only one of two or more 
consocies; for example, Typha may alone represent the three usual consocies 
of the reed-swamp. Particular consocies may be indicated by using a suffix 
with the generic name, as in the case of consociation. It is proposed to employ 
the suffiix -ies for this purpose, as in Scirpies, Typhies, Phragmities, Aristi- 
dies, etc. 

Socies. — The socies bears exactly the same relation to consocies and asso- 
cies that the society does to consociation and association. It is a serai society, 
characteristic of a developmental community instead of a climax one. It is 
marked by subdominance within the dominance, in the way that a society is 
composed of a subdominance within a climax dominance (plate 17 b). 

Socies show the same differences as those found in societies. They are more 
or less characteristic of aspects, and they occur in layers, though to a smaller 
extent, since layers are well-developed only in the final stages of a sere. 
Cryptogamic or thallus socies are especially numerous, since such communities 
are characteristic of many initial stages. As is evident, the term socies 
comes from the root seq- {sec-, soc-), follow, found in its primary or secondary 
meaning in all the preceding terms. While the prefix con- in consocies indi- 
cates the grouping of serai dominants to form an associes, its absence in socies 
suggests the fact that the latter are not exact subdivisions of the consocies. 
Socies are designated by using the generic name with an affix, as in the case 
of the society. In place of the locative suffix, -He, the diminutive -trie is pro- 
posed, as in Sedule, Violule, Silenule, etc. This has the advantages of at 
least suggesting the earlier serai position of the socies with reference to the 
society, and of indicating by the similarity of the two suffixes the close rela- 
tionship between the respective communities. 

Colony. — The colony is an initial community of two or more species. It is 
practically always a direct consequence of invasion, and hence is character- 
istic of the early serai development in bare areas. It may arise from the 
simultaneous entrance of two or more species into the same spot, or it may 
result from the mingling of families. From their occurrence in bare areas, 
particular colonies are nearly always sharply delimited. They may appear in 
the midst of later dense communities whenever a minute bare spot permits 
invasion, or whenever success in competition enables an invader to make a 
place for itself. In such places they simulate clans, but can be readily dis- 
tinguished by a careful scrutiny. 



140 STRUCTURE AND UNITS OF VEGETATION 

Colonies resemble clans in their usually limited size and in the absence of 
a clear relation to the habitat, because they are still in the process of invasion. 
They differ in appearing normally in bare areas or in open vegetation and 
in being developmental in character. A colony differs from a family in con- 
sisting of two or more invaders instead of one. It is one of the two kinds of 
community formerly recognized by Clements (1905:297; 1907:227, 239). A 
colony does not have a fixed rank, but it may develop later into any com- 
munity of higher rank in the developmental series. As already indicated, 
it is primarily a mixed invasion group, which is inevitably worked into the 
history of the sere as development proceeds. The term colony is itself an 
index of this pioneering quality. Colonies may be designated by the suffix 
-ale, as in Hordeale, Ambrosia-Ivale, etc. 

Family. — The use of the term family for an ecological group was proposed 
by Clements (1904:297, 299; 1905:297; 1907:228, 237). The fundamental 
identity of such families of plants with those of animals and man is thought 
to make such use of the word unavoidable in spite of the established usage 
for a systematic unit. While the possibility of confusion from the double 
use of the term is slight, it may prove desirable to avoid this objection alto- 
gether by using the term famile for the ecological unit. As is evident, it is 
from the same root as family, and has essentially the same meaning. 

A family is a group of individuals belonging to one species. It often springs 
from a single parent plant, but this is not necessarily the case, any more than 
in a human family. It may consist of a few individuals or may extend over 
a large area. The group of cells within a Gloeocapsa sheath is a family, and 
not a colony in the proper sense. The coating of Pleurococcus on a tree-trunk 
is a family, as is also a tuft of Funaria at its base, or the group of Helianthus 
which fills a large field to the exclusion of all other flowering plants. Families, 
however, are usually small, since they are more readily invaded when large, 
and consequently pass into colonies. They are especially typical of bare 
areas and initial stages. They rarely appear in dense vegetation, except where 
local denudation occurs. As the individuals of a family become more numer- 
ous, adjacent families merge into a colony ; or migrules from one family may 
invade another at some distance and convert it into a colony. Since the family 
always consists of a single species, it may be designated in the usual way by 
adding the patronymic suffix to the generic name, as in Sedas, Aletas, Erio- 
gonas, etc. Where greater definiteness is desired, the specific name in the 
genitive form may be added, e. g., Buhas strigosi. 

Summary of units. — The following table is intended to show the relation 
of climax and serai or developmental units to the formation, the relation of 
the units of each series to each other, and the correspondence of units in the 
two series. 

Formation. 
Climax Units : Serai Units : 

Association Associes. 

Consociation Consocies. 

Society Soeies. 

Clan Colony. 

Family. 

Mixed communities. — Clements (1905:304; 1907:235) has considered 
briefly the mixing of communities as a consequence of juxtaposition or of 



CLEMENTS 




w* 






™ 1 



.*»*.*/' 



^y** |lv : S^J*?^' 



' -^AfP: > 



V;. 



.^■;.r : " i 



V 




A. Art< mixia-Pointlus-ccotaiit , Fallon, Nevada. 
B. Picea-Populus-mictium, Alpine Laboratory, Colorado. 



NOMENCLATURE OF UNITS 141 

succession. The former applies to the characteristic mingling of dominants 
where their corresponding communities touch. It may occur between two or 
more formations, associations, consociations, or societies, or between associes, 
consocies, or socies. In every case the mixing takes place at the borders of the 
communities concerned, producing an ecotone or tension. This is often very 
extensive, and frequently its relations are very puzzling. Difficult as the task 
may be, however, the real nature and significance of such an ecotone can be 
determined only from a study of the adjacent communities. 

The mixing of two stages in development is much more complex and puz- 
zling. This is due to the fact that mixing may take place throughout the area 
and in varying degree. There are consequently in such places no distinct 
areas of the two stages with which comparison can be made. Hence it is 
necessary to turn to other examples of the same development and to make a 
comparative study extending over a wide region and over several years. While 
there is inevitably some mixture in all stages of the sere, it is only when the 
dominant species of two, or rarely more, stages are present on somewhat of 
an equality that a real mixture may be said to result. It is now proposed to 
restrict the term mictium (Clements, I. c.) to this developmental mixture, and 
to use ecotone for an actual transition area on the ground between two com- 
munities, regardless of whether the latter are climax or serai. Thus, a 
Populus-Pinus-mictium is a more or less uniform mixture of two successive 
consocies, while a Populus-Pinus-ecotone is a band of mixed aspen and pine 
between two pure communities of each (plate 18, a, b). 

Nomenclature of units. — The whole task of ecological nomenclature is to 
secure a maximum of characteristic with the minimum effort. A long step 
toward this result is taken by having a definite concept and name for every 
distinct unit. The method of suffixes, first used by Schouw (1823:65) in 
designating groups by adding -etum to the generic name, has furnished the 
model for the designation of all groups in which life-form and dominance are 
the chief characteristics. Such are the consociation, consocies, society, etc. 
Where the habitat is of primary importance, as in the formation, association, 
and associes, it is necessary to employ a separate word, poion, helion, hylion, 
eremion, etc., to indicate it. This must then be qualified by the use of the 
generic name for actual floristic definiteness (Clements, 1902:16). Difficul- 
ties arise, however, when two or more genera are concerned, or when it is 
necessary to indicate the species in order to secure the requisite definiteness. 
In both cases a balance must be struck between usability and definiteness, and 
the latter must often be sacrificed. In the case of the Great Plains grassland, 
definiteness would demand that it be termed the Bouteloua-Bulbilis-Aristida- 
poion. Such a name is impracticable, as taxonomy long ago proved in the case 
of polynomials. The use of two generic names is the most that convenience 
permits, and one is better still. In the ease cited, since Bouteloua is the domi- 
nant of the widest range and greatest importance, the grassland might well 
be called the Bouteloua-poion. Once the names of units become generally 
recognized, such a designation is no more indefinite or incomplete than 
Solanacece, for example. 

In this connection, Moss (1910:41) states that: 

"The naming of a pure association [consociation] by its dominant species 
is comparable with the plan of naming a systematic group after an easily 



142 STEUCTURE AND UNITS OF VEGETATION 

recognizable character; and in neither case does such a name exhaust the 
characters of the group it denotes." 

This statement does not seem wholly consistent with the further statement 
that: 

"This name, Eriophorum-Scirpus-Oxodion, would not, however, be really- 
definitive, as no indication would be given of the species of Eriophorum or 
Scirpus, which are the dominant plants of the two associations ; and the range 
of habitat and of form of these two genera is considerable. Nor do such terms 
as 'magna-Caricetuni' and 'parvo-Carieetum' (Schroter, 1904:49) overcome 
this difficulty in the least. In the British Isles alone there are, in this forma- 
tion, associations of Calluna vulgaris, of Empetrum nigrum, of Eriophorum 
angusti folium, of E. vaginatum, of Molinia coerulea, of Vaccinium myrtillus. 
and others. Add to these the various other associations known and described 
on the continent of Europe alone, and the designation of the formation by 
Clements' plan reaches Brobdingnagian proportions." 

While no such sesquipedalian terms were contemplated in the plan men- 
tioned, the criticism loses its weight in the case of the developmental classifica- 
tion of formations as climax units. Each formation would rarely contain more 
than two or three associations, and it is merely a question of a compromise 
between securing the necessary brevity and the desired definiteness. Where 
the generic names of the chief dominant of each association are short, two or 
three such names might be used with maximum definiteness and little incon- 
venience. As a rule, however, two names alone would be permitted by the 
demands for brevity, and often one would be better still. Once in use, Boute- 
loua-poion, Stipa-poion, or Picea-hylion would be no more indefinite than 
Solanaeeae, Rosaceae, etc. It seems such a designation of the formation would 
have a distinct advantage over the proposal to designate the various climatic 
formations as <*-Oxodion, /3-Oxodion, etc. (Moss, 1910:44). In the case of 
mixed communities, definiteness demands the use of the two chief dominants, 
whether they are consociations as in an ecotone or consocies as in a mictium. 

Hult (1881:22) was the first to propose and use a system of nomenclature 
for formations. He considered the use of names based upon the habitat to 
be impossible, for the reason that the same formation [community] might 
occur in quite different habitats. Hence he found it necessary to propose an 
entirely new nomenclature, modeled after Kerner, in which formations were 
named from their characteristic vegetation-forms. As he understood it, the 
pine formations contained three such forms, the Pinus-iorm, Myrtus-f.ovm, 
and Cladina-lorm, and hence were termed "pine and lichen formations," 
Pineta cladinosa. Hurt's evident intention was to form a binomial nomencla- 
ture based upon that of taxonomy, an attempt which has much to commend it 
theoretically. Practically it results too often in a lack of definiteness and 
brevity, produces an endless series of names, and fails completely to indicate 
developmental relations. Such names as Pineta cladinosa, Betuleta muscosa, 
and Aireta geraniosa are attractive, but Oeranieta graminifera, Aireta herbida 
and Aireta pura are ambiguous and confusing, while Sphagneta schoenola- 
gurosa, Juncelleta polytrichosa, Pseudojunceta amblystegiosa, and Grandi- 
cariceta amblystegiosa are quite too long and indefinite. 

Cajander (1903:23) has proposed to designate associations (consociations) 
more exactly by using the genitive of the species with the generic name in 



FORMATION GROUPS 143 

-etum, e. g., Salicetwm, Salicis viminalis, though in use this becomes Salicetum 
viminalis, Alnetum incanae, etc. Moss (1910:41) adopts this plan, and Warm- 
ing (1909 :145) apparently approves it also. As a consequence, it may well be 
generally adopted in all cases where such definiteness is desired. In the actual 
consideration of a consocies or other unit it would seem unnecessary and in- 
convenient to repeat the full form, e. g., oxodion Eriophoreti vaginat\ 
Aristidae purpureae pois, etc. The full form once given, Eriophorum-oxodion 
or Aristida-pois would meet all requirements, except where actual confusion 
may arise when there are two dominants of the same species in one associa- 
tion or associes. 

The names of units are necessarily long at best, and it seems both desirable 
and justifiable to shorten them in every legitimate way. The most efficient 
way of doing this is the one already suggested, namely, of using the name of 
the chief genus or even a characteristic genus alone in the case of formation, 
association and associes, exactly as has been found so successful in the case of 
families and orders. In the case of terminations in particular there can be no 
valid objection to the use of shortened stems and of the contraction or elision 
of successive vowels. The classical purist will find the former method objec- 
tionable, but the fact remains that it was in use by classical writers themselves. 
A study (Clements, 1902:31) of Greek neuters in -jua-ros, nom. -pa, e. g., 
sperma, stoma, etc., has shown that some of them occur usually in this form, 
and still more take this form frequently. In their use in biology, Greek and 
Latin must be regarded as living languages and hence subject to change 
along the lines already indicated. Hence there is the warrant of brevity and 
convenience as well as of actual classical practice for the shortened forms 
found in Spermophyta, Dermocyoe, stomal, etc. This usage may well be 
extended to other imparisyllabic stems in -idis, -itis, etc. Thus Calamagrosti- 
detum would become Calamagrostetum; Heleocharitetum, Heleocharetum; 
Lychnidetum, Lychnetum. Such abbreviations have already been made, 
though it is very doubtful whether such extreme cases as the shorten- 
ing of Potamogetonetum to Potametum are to be approved. The con- 
traction or elision of vowels especially is often desirable also, even though 
the gain is small. The chief gain is in pronunciation rather than in 
spelling as Picetum for Piceetum, Hordetum for Hordeetum, and Spiretum 
for Spimeetum. 

Formation groups. — The arrangement of formations into higher groups 
has been based upon various grounds. The first systematic grouping was that 
of Schouw (1823:157), who used the amount and nature of the water-content 
to establish the four generally accepted groups hydrophyta, mesophyta, xero- 
phyta and halophyta, though he named only the fii'st and last. The term 
xerophyte or xerophilous dates back to Thurmann (1849) and mesophyte to 
Warming (1895), who adopted Schouw 's classification in essence. Drude 
(1890:37) classified formations as (1) forest, (2) grassland, (3) swamp 
moor, (4) miscellanous, rock, water, and saline. Pound and Clements (189S: 
94; 1900:169) adopted Warming's divisions, but subdivided mesophytes into 
hylophytes, poophytcs, and aletophytcs. Sehimper (1898), while recognizing 
water-content groups, classified formations with respect to life-form as forest, 
grassland, and desert, and with regard to habitat as climatic and edaphic. 
Graebner (1901:25) grouped formations on the basis of soil solutes into those 



144 STRUCTURE AND UNITS OF VEGETATION 

on (1) pernutrient, (2) enutrient, (3) saline soils. Cowles (1901:86) used 
physiography and development for the basis of the following groups: (A) 
Inland group: (1) river series, (2) pond-swamp-prairie series, (3) upland 
series; (B) Coastal group: (1) lake-bluff series, (2) beach-dune-sandhill 
series. Clements (1902:13) arranged formations in various groups, based 
upon medium, temperature, water-content, light, soil, physiography, physiog- 
nomy, association, and development. Schroter (1902:73) proposed two major 
groups: (1) vegetation type, subdivided into (2) formation groups, and the 
latter into formations. Grassland is given as an illustration of the vegetation 
type, and meadow of the formation group. Clements (1904:139; 1905:302, 
270) arranged formations with reference to habitat, development, and region, 
but emphasized the developmental classification as primary. Warming and 
Vahl (1909 :136) propose 13 classes of formations on the basis of climatic and 
edaphic distinctions. To the original 4 groups of Warming are added helo- 
phytes, oxylophytes, psychrophytes, lithophytes, psammophytes, chersophytes, 
eremophytes, psilophytes, sclerophyllous, and coniferous plants (c/. Clements; 
1902:5). Brockmann and Biibel (1913:23) have recognized three major 
groups: (1) vegetation type, (2) formation class, and (3) formation group. 
For example, the vegetation type, woodland or "Lignosa," is divided into sev- 
eral formation classes, e. g., " Pluviilignosa, " " Deciduilignosa, " etc., and 
these into groups, such as " Aestatisilvae, " " Aestatif ruticeta, " etc. The pri- 
mary basis of the classification is physiognomy, with some reference to habitat 
in many of the classes and groups. 

Bases. — A comparison of the various systems proposed above shows that 
there are three general bases. These are habitat, physiognomy, and develop- 
ment. These practically exhaust the list of possibilities, since floristic does 
not furnish a feasible basis. All systems based primarily upon habitat make 
use of physiognomy in some degree, and the converse is also true. They do 
not take development into account, and hence are more or less superficial. 
The simplicity and convenience of artificial classifications based upon habitat 
and physiognomy are so great, and the readiness with which they can be made 
is so alluring, that they will persist for a long time. They must slowly yield 
to a natural system based upon development, but such a system in its details 
demands much more knowledge of vegetation and climate than we possess at 
present. There can be no serious objection to using a habitat-physiognomy 
or a physiognomy -habitat system in so far as it is useful and accords with the 
facts. It should be constantly borne in mind, however, that such classifica- 
tions are makeshifts against the time when developmental studies have 
become general. 

Developmental groups. — The formation as generally understood is based 
in no wise upon development. Hence the natural or developmental relation 
of such formations or associes, as they are called here, is revealed by the 
physiographic classifications of Cowles (1901). Such a system broadened 
to become purely developmental, and with physiography regarded as but one 
of several causes is the one which we have already considered in various 
aspects. The formation as here conceived is a natural unit in which all of 
its associes, the formations of many authors, fall into their proper develop- 
mental relation. It has already been pointed out that such a relation includes 
all the essentials of habitat and physiognomy. 



DEVELOPMENTAL GROUPS 145 

The classification of formations, i. e., climax communities, as here under- 
stood, is a more difficult task. Here again the fundamental basis should be 
that of development, but we now have to do with the phylogenetic develop- 
ment of a climax formation, and not with its ontogeny. The ontogenetic 
development of a formation, such as the Great Plains grassland, can be 
studied in hundreds of primary and secondary seres. Its phylogeny is a 
matter of the past. It not only can not be studied with profit until the 
present development is well understood, but it must always remain partly a 
matter of speculation. It is only in the case of fairly complete records, such 
as those of peat-bogs, that the actual origin of a climax formation can be 
traced. From their very nature such formations are dependent upon climate. 
This fact furnishes the best basis for a natural classification at present. In 
this connection it is instructive and convenient to group the climaxes of 
similar climates together, as for, example, the plains of America and the 
steppes of Eurasia. Such a classification emphasizes the essential relation of 
climax and climate, but is not necessarily genetic. Such a genetic or develop- 
mental classification can be based at present only upon the regional relation 
of climaxes, as indicated in Chapter IX. A system of this sort is suggested 
by the regional classification of Clements (1905:304), in which (1) lowland, 
(2) midland, (3) upland, (4) foot-hill, (5) subalpine, (6) alpine, and (7) 
niveal formations correspond closely to a similar series of climaxes, namely, 
(1) deciduous forest, (2) prairie, (3) plains, (4) scrub, (5) montane forest, 
(6) alpine grassland, (7) lichen and moss tundra. A similar relation exists 
in the case of continental zones of temperature {I. c, 283), the (1) polar- 
niveal, (2) arctic-alpine, (3) boreal-subalpine, (4) temperate, (5) subtrop- 
ical, and (6) tropical zones, corresponding essentially to as many climatic 
climaxes, more or less interrupted by the superimposed series indicated 
above. 

The sequence of climates and climaxes in either of the above series indicates 
the course of development in the event of any normal climatic change. If the 
climate of the Mississippi basin becomes drier, prairie will encroach upon and 
replace deciduous forest, and the plains will conquer prairie to the east and 
scrub to the west, etc. If the rainfall increases the deciduous forest will 
extend more and more into the prairie, the latter will move westward over the 
plains, and the plains will be further narrowed by the creeping out of scrub 
and forest from foot-hills and mountains. The appearance of another period 
of glaciation would produce a similar shifting of climaxes. The polar-niveal 
climax would move into the arctic-alpine climax, the latter into the boreal- 
subalpine climax, etc., the amount of movement and replacement depending 
upon the extent and duration of the ice. The reverse migration of climaxes 
would occur upon the melting of the ice-sheet, as it must have occurred at 
the end of the glacial epoch, and to a certain extent in interglacial intervals. 
A climatic series of climaxes or formations is an epitome of past and potential 
development, i. e., of phylogeny. It is both genetic and natural, and furnishes 
the basis for a natural classification of climax formations. Such a series is 
the connecting link between the coseres of one climatic period and another, 
that is, between two different vegetation periods geologically speaking, or 
eoseres. Prom its step-like nature and its relation to climate and climax. 
such a regional-historical series may be termed a disc re. This term is formed 

li 



146 STRUCTURE AND UNITS OF VEGETATION 

by combining sere with the unmodified root cli, found in Gr. kAiVw, make 
to bend or slope, aXi^a, slope, region, climate, and Lat. clivus, slope, hill. In 
accordance with what has been said above, it is here proposed to group 
formations in climatic series or cliseres. The illustrations already given would 
constitute two cliseres, one dependent upon water primarily, the other upon 
temperature. Cliseres in turn would be related to definite eoseres. 



VIII. DIRECTION OF DEVELOPMENT. 

Development always progressive. — Succession is inherently and inevitably 
progressive. As a developmental process, it proceeds as certainly from bare 
area to climax as does the individual from seed to mature plant. While the 
course of development may be interrupted or deflected, while it may be slowed 
or hastened, or even stayed for a long period, whenever movement does 
occur it is always in the direction of the climax. In this connection, however, 
it is imperative to distinguish between the development of the sere and the 
action of denuding agencies. This is particularly necessary when such a 
process as erosion acts with varying intensity in different portions of the same 
area. At first thought it seems permissible to speak of such a community as 
degenerating or retrograding. A closer analysis shows, however, that this is 
both inaccurate and misleading. What actually occurs is that the community 
is being destroyed in various degrees, and secondary areas of varying char- 
acter are being produced. In these, colonies appear and give a superficial 
appearance of regression, but in no case does actual regression occur. In 
every denuded area, no matter how small, development begins anew at the 
stage determined by the degree of denudation, and this development, as 
always, progresses from the initial colonies to or toward the climax 
formation. 

Nature of regression. — Regression, an actual development backwards, is 
just as impossible for a sere as it is for a plant. It is conceivable that lumber- 
ing, grazing, and fire might cooperate to produce artificial regression, but 
there is nowhere evidence that this is the case. Apparent regression would, 
and probably does, occur when the forest canopy is removed by the ax and the 
shrub layer is also later removed as a consequence of grazing or fire, permit- 
ting the final establishment of herbland or grassland. Here, however, there 
can be no question of development, for the whole process is one of destruction, 
of partial denudation. The consocies resemble those of the final stages of the 
original sere, but are largely or wholly different as to the constituent species. 
The actual condition is one characterized by the removal of the dominants and 
the consequent change of the controlling conditions. The latter results in the 
disappearance of many principal and secondary species and the concomitant 
invasion of new ones. As long as the artificial forces which brought this about 
persist or recur, the community will be held in a subclimax, i. e., the develop-* 
ment is checked in much the same way that extreme cold or wet stops the 
growth of the individual plant. Once the inhibiting forces are removed, 
normal development is slowly resumed and progresses to the proper climax, 
provided the climax community still persists in adjacent areas. 

The apparent exception afforded by the Sphagnum invasion of grassland or 
woodland communities is discussed a little later. Here again a close scrutiny 
of the facts indicates that this is but another ease of local and partial denuda- 
tion due to water. The case is complicated by the fact that the growth of 
Sphagnum is both a cause and a consequence of the increased water and of 
the resulting denudation by overwhelming or flooding. Successionally, 
Sphagnum stands in the same causal relation to the flooding that a beaver- 

147 



148 DIRECTION OF DEVELOPMENT 

dam or local surface erosion does. It is both a cause and a pioneer, however, 
and this dual role has tended to conceal the essential relation. 

Course of development. — The basic and universal progression from bare 
areas to climax is a complex correlated development of habitat, community, 
and reaction. The general relation of these is indicated by the gradual coloni- 
zation of a bare area, and the progression of associes until the climax is 
reached. Beneath this, as motive forces, lie invasion and reaction. The total 
effect is seen in four progressive changes or processes. The initial change 
occurs in the habitat, which progresses normally from an extreme or rela- 
tively extreme condition to a better or an optimum condition. This is espe- 
cially true of the unfavorable extremes of water-content, both as to quantity 
and quality, i. e., the presence of alkali, acid, etc. With respect to the plant 
life, the progressive movement is from lower to higher phyads, from algae, 
lichens, or mosses, to grasses and woody plants. The interaction of habitat 
and community results in a progressive increase of dominance and reaction, 
both in the most intimate correlation. Finally, in the climax formation as a 
whole, there is the simultaneous progression of almost innumerable primary 
and secondary seres, all converging toward the climax into which they merge. 

Regression and retrogression. — It has already been stated that regressive 
development is impossible and that regressive succession does not occur. 
Hence it becomes necessary to examine the views of several authors who dis- 
tinguished between progressive and regressive succession and to interpret 
their observations in terms of development. Such a distinction seems first 
to have been made by Nilsson (1899) in the study of the development of 
Swedish associations. Cowles (1901) used the same terms, but with a very 
different meaning, in his physiographic treatment of the ecology of the region 
about Chicago. Cajander (1904) adopts the distinction proposed by Nilsson, 
as does Moss also (1910), while Hole (1911) uses progressive and regressive 
in still another sense. As will become evident, some of these concepts are sub- 
jective and have little relation to the organic development, while others rest 
upon an incomplete interpretation of the facts. The existence of five conflict- 
ing views seems to afford illuminating evidence as to the actual occurrence 
of such a distinction in nature. 

Nilsson 's view. — The regular development of vegetation about the lakes 
of Sweden exhibits the following stages: (1) sedge moor; (2) Eriophorum 
moor; (3) scrub moor, with various consocies, chiefly Calluna, Erica and 
Betula; (4) forest moor, usually Pinus silvestris, often Picea or Betula. This 
is properly regarded as progressive development. Regressive development is 
said to take place when lichens, Cladina and Cladonia, appear in the Sphag- 
num masses of the scrub moor, and come to play the dominant role, as the 
Sphagnum and shrubs die off in large measure. Sphagnum, Eriophorum, and 
the shrubs still persist, however, in scattered alternes. The cause of the 
regression lies in the drying-out of the upper layer, the death of the Sphag- 
num, and the consequent weathering of the peat. During wet seasons the 
lichens perish through the accumulation of water. Eriophorum and Andro- 
meda persist longer, but finally die out also as a result of the continued 
accumulation of water. The water is colonized by Sphagnum and sedges, 
especially Car ex limosa, Scirpus caespitosus or Scheachzeria, and progressive 
development begins anew, to terminate in forest moor or to be again inter- 



REGRESSION 149 

rupted by unfavorable conditions. This continues until progressive develop- 
ment prevails throughout the entire area, and finally terminates in the climax 
forest. The development rarely proceeds uniformly over a large area, but 
progressive and regressive areas alternate with each other constantly. (371) 
Nilsson's whole description agrees perfectly with the course of events in a 
sere where local conditions bring about the destruction of a particular stage 
in alternating spots. He makes it clear that drying-out kills the Sphagnum, 
Eriophorum, and shrubs in certain areas, and produces conditions in which 
lichens thrive. In turn, the accumulation of water kills the lichens, and, more 
slowly, the relict Eriophorum and shrubs, and prepares a new area for the 
invasion of Sphagnum and sedges. In all of this destruction there is nothing 
whatever of an organic successional development. Wherever plants are 
destroyed, whether quickly or slowly, over large areas or in a spot of a few 
square centimeters, invasion becomes possible, and local development begins. 
A general view of a moor with alternating pools and hummocks, of drier and 
wetter places, may well give the appearance of regression. But this is an 
appearance only, for in each pool and on every hummock development pro- 
ceeds always in a progressive direction, though it may be interrupted again 
and again by a change of conditions. Nilsson also regards the repeated 
passage from progressive to regressive to progressive again as indicating a 
circulation or cycle of development, but this view depends upon the existence 
of an actual backward development. 

Cowles's view. — Cowles distinguishes between progressive and retrogres- 
sive succession chiefly upon physiographic grounds. The distinction is drawn 
clearly in the following statement: 

"Retrogressive phases, i. e., away from the mesophytie and toward the 
hydrophytic or xerophytic, must be included, as well as progressive phases 
away from the hydrophytic and toward the mesophytie." (81) 

The distinction is further elaborated as follows: 

"In flood plains, the meanderings of the river may cause retrogressions to the 
hydrophytic condition, such as are seen in oxbow lakes, or the river may lower 
its bed and the mesophytie flood plain become a xerophytic terrace. The 
retrogressive phases are relatively ephemeral, while the progressive phases 
often take long periods of time for their fidl development, especially in their 
later stages. ... If a climate grows colder or more arid, we find retro- 
gressive tendencies toward the xerophytic condition, while in a climate that 
is getting more moist or more genial, the mesophytie tendencies of the erosive 
processes are accelerated. . . . (82) Retrogression is almost sure to come 
in connection with terrace formation. A river may swing quite across its flood 
plain, destroying all that it has built, including the mesophytie forest. Not 
only is the vegetation destroyed directly b\it also indirectly, since the lowering 
of the river causes the banks to become more xerophytic. . . . (107) 
The life history of a river shows retrogression at many points, but the progres- 
sions outnumber the retrogressions. Thus a river system, viewed as a whole, 
is progressive. . . . (108) A young topography is rich in xerophytic 
hills and in hydrophytic lakes and swamps. There may be local retrogressions 
toward xerophytic or even hydrophytic plant societies, forming eddies, as it 
were, but the great movement is ever progressive and toward the mesophytie 
condition. So far as plants are concerned, however, a physiographic termi- 



150 DIRECTION OF DEVELOPMENT 

nology may be still used, since all possible crustal changes are either toward or 
away from the mesophytic, i. e., progressive or retrogressive." (178) 

In connection with succession on dunes, Cowles states that: 

"A slight change in the physical conditions may bring about the rejuvena- 
tion of the coniferous dunes, because of their exposed situation. This rejuven- 
ation commonly begins with the formation of a wind sweep, and the vegetation 
on either hand is forced to succumb to sand-blast action and gravity." (174) 

Elsewhere (172) the dune complex is described "as a restless maze, advanc- 
ing as a whole in one direction, but with individual portions advancing in 
all directions. It shows all stages of dune development and is forever chang- 
ing. ' ' Such destruction of existing communities and the production of a bare 
area are essentially the same as the changes in moor which Nilsson calls regres- 
sive. Cowles nowhere applies this term to the dune sere, and appears in no 
place to speak of the succession or development as retrogressive. Indeed, the 
use of the word "rejuvenation" in this connection is a fortunate one, as it 
emphasizes the essentially reproductive nature of the developmental process. 

The use of progressive and retrogressive in connection with the development 
of seres in river valleys illustrates the undesirability of transferring physio- 
graphic terms to the organic development of vegetation. It is evident that 
a river system shows almost constant, though more or less local retrogression 
throughout its general progressive development during a single cycle of ero- 
sion. The bed of a river, its banks, flood-plain, and terraces are constantly 
reshaped by erosion and deposition in conformity with a general law. The 
material of the land is not destroyed, but merely shifted. Such is not the case 
with the community which occupies an area of erosion or deposit. As shown 
above, Cowles points out in such cases that the vegetation is destroyed directly 
or indirectly. Hence there can be no such thing as retrogression in the suc- 
cessional development. What does occur is the universal phenomenon of suc- 
cession, in which one serai development is stopped by the destruction of a 
particular stage, and a new sere starts on the bare area thus produced. If 
the term "retrogressive phase" be applied solely to the usually brief period 
when the community is being destroyed, it fits the facts, but is still misleading. 
It implies a backward development or devolution comparable to the progres- 
sive evolution or development of the sere, while as a matter of fact it applies 
not to development but to its complete absence, i. e., to destruction (c/. 
Crampton, 1911:27; Crampton and MacGregor, 1913:180). 

The difficulty of distinguishing progression as movement toward a meso- 
phytic mean and retrogression as movement away from it is well illustrated 
in succession in desert regions. The development of vegetation in a desert lake 
or pond passes from hydrophytic to mesophytic, and then to xerophytic or 
halophytic stages. Organically this is a unit development from a bare area 
to a climax community. Yet the distinction just mentioned would require 
that it be divided into progression and retrogression. The only possible retro- 
gression is in the decreasing water-content, and yet this decrease of water- 
content is a constant feature of the progression from ordinary water areas 
to mesophytic conditions. Similarly, the successional development along the 
coast of the Philippines would present a peculiar difficulty, if Whitford's 
interpretation is correct (1906:679). He regards hydrophytic forest as the 
climax, and the entire development would consequently be retrogressive. 



REGRESSION 151 

In a later paper (1911:170), Cowles appears indeed to regard retrogression 
as little if at all different from destruction, and to interpret physiographic 
processes chiefly in terms of destruction and development or progression. 

"As might be expected, the influence of erosion generally is destructive to 
vegetation, or at least retrogressive (i. e., tending to cause departure from the 
mesophytic), while the influence of deposition is constructive or progressive 
{%. e., tending to cause an approach to the mesophytic). On a somewhat 
rapidly eroding clay cliff of Lake Michigan ... a marked increase in erosive 
intensity would destroy all vegetation, and a marked decrease in erosive 
intensity might institute a progressive vegetative succession. Frequently 
a growing dune is inhabited by xerophytic annuals and by a few shrubs or 
trees ; such a place illustrates the extreme of topographic dynamics, but often 
the vegetation is static. A great increase in depositional intensity results in 
the destruction of all the plants, while a decrease in depositional intensity 
results in progressive succession." 

Cajander's view. — Cajander (1904) has studied three moors of northern 
Finland in connection with Nilsson's concept of progressive and regressive 
development, and has reached the conclusion that these views are correct. 
Moor I is regarded as in the course of primary progressive development 
characterized by a continuous fresh green moss layer, with low and indefinite 
cushions of heath-moor. Moor II is assumed to be chiefly in regressive 
development, as it is made up of areas of cyperaceous moor separated by strips 
of heath-moor. The reasons for this view are that (1) many areas are bare 
spots of decayed turf, (2) the sedge areas are often sharply delimited and 
raised above the heath-moor areas, (3) the moss-layer is lacking or consists of 
other mosses than Sphagnum. In the extensive Moor III, regressive develop- 
ment has everywhere taken place, and cyperaceous communities occur through- 
out. In a large part of the moor the regressive development is followed by a 
secondary progressive development, and in small areas of the latter is found a 
secondary regressive development. On these grounds the author regards the 
view of Nilsson that there is a "circulation" or cycle in the development 
as well-grounded. As already pointed out in this connection, regressive 
development is only destruction or denudation followed by the normal devel- 
opment, which is always and inevitably progressive. Denudation or destruc- 
tion may recur again and again at any stage of succession in many separate 
areas of the community and hence produce a maze of so-called "regressive" 
and progressive areas. 

Sernander's view. — Sernander (1910:208) has drawn a distinction between 
progressive and regenerative development : 

"The real cause why the Sphagnum peat is heaped up in such fashion lies 
in the fact that the moribund parts lag behind the living Sphagnum in growth, 
and finally form hollows in the latter. These hollows fill gradually with water, 
while the erosion of the surrounding peat-walls increases their extent. In the 
water arise new Sphagncta, which begin in miniature the progressive develop- 
ment, which I term regeneration. This regenerative development of the hol- 
lows soon culminates in CaUuna-hcath or is interrupted by a new formation 
of hollows. The latter develops in the usual way, and in this manner arises 
one lens-shaped peat-mass above another, characterized above and below by 
dark streaks, usually of heath-peat." 



152 DIRECTION OF DEVELOPMENT 

In discussing the origin of the high moor of Orsmossen (1910:1296) Ser- 
nander states that: 

"After the progressive development, where regeneration plays a relatively- 
minor role, appears a stage in which the moor passes simultaneously into 
heath-moor over large areas with uniform topography. (In the deeper hol- 
lows, the progressive development may proceed further.) In the sequence 
of the layers, the lower Sphagnum peat is followed by a more or less coherent 
layer of heath peat. With the development of the heath moor begins the 
formation of hollows, and the accumulation of regenerative peat masses, 
commonly with great sods of Andromeda-Sphagnum peat and Scheuchzeria- 
Sphagnum peat directly above the peat of the heath moor." 

Sernander's description of the formation of hollows by the death of the 
peat and of the consequent production of tiny pools which are invaded by 
Sphagnum furnishes outstanding proof that the retrogressive development of 
Nilsson and Cajander is actually the death of a plant community or a part of 
it, and the resulting formation of a bare area for colonization. No serious 
objection can be brought against the use of the term regeneration or regenera- 
tive development, and it has the advantage of being in harmony with the idea 
that succession is a reproductive process. It does, however, obscure the fact 
that the development is nothing but the normal progressive movement typical 
of succession. It is normally secondary, but, differs from the primary progres- 
sive development only in being shorter and in occurring in miniature in hun- 
dreds of tiny areas. 

Moss's view. — Moss (1910:36) makes the following statements in regard 
to the direction of movement : 

"Succession of associations within a formation may be either progressive 
or retrogressive. In the salt marshes in the south of England for example, 
a succession of progressive associations of Zostera, of Spartina, of Salicornia, 
etc., culminates in a comparatively stable association of close turf formed of 
Glyceria maritima. The latter association, however, may be attacked by the 
waves and ultimately destroyed; and thus retrogressive associations are pro- 
duced. In the case of established woods, we do not know the progressive 
associations which culminated in the woodland associations ; but we can deter- 
mine retrogressive stages through scrub to grassland. Similarly, the retrogres- 
sive associations which are seen in denuding peat moors are recognizable. 

"A plant formation, then, comprises the progressive associations which 
culminate in one or more stable or chief associations, and the retrogressive 
associations which result from the decay of the chief associations, so long as 
these changes occur in the same habitat. 

"It sometimes happens, as in the case of the peat moors on the Pennine 
watershed, that the original habitat is wholly denuded and a new rock or soil 
surface laid bare. In other cases, as when sand-dunes are built up on the site 
of a pre-existing salt marsh, a habitat may be overwhelmed by a new one. 
In such cases the succession passes from one formation to another formation. 
Again, a new habitat is created when an open sheet of water is choked up with 
silt and peat. 

' ' Every formation has at least one chief association ; it may have more ; 
and they may be regarded as equivalent to one another in their vegetational 
rank. They are more distinct and more fixed than progressive or retrogressive 
associations. Open progressive and retrogressive associations, however, fre- 



CLEMENTS 




A. Denudation in moorland, the peat-hags capped here ami there with bilberry (Vaccinium 

myrtillus) ; "retrogression" of the cotton-grass moor (Enophoretum). 

B. Degeneration of beechwood duo to rabbits, Holt Down. Eanipshire, England. 



REGRESSION 153 

quently occur in formations whose chief associations are closed. Unless, how- 
ever, the progressive and retrogressive associations are included in the same 
formation as the chief associations, an incomplete or unbalanced picture of the 
vegetation results." 

In the first paragraph the real identity of retrogression with destruction 
and denudation is clearly indicated by the author in the statements that the 
stable association of Glyceria maritime/, may be destroyed by the waves, and 
that retrogressive associations are recognizable in denuding peat-moss. More- 
over, he ignores the part light plays in determining habitat limits, and conse- 
quently the normal developmental relation of reaction to changes of popula- 
tion. The production of new areas by denudation and by deposition is dis- 
tinctly pointed out, but the essential correlation of this with succession is 
not made (plate 19 a). 

The views of Moss were adopted by Tansley and several of his associates, 
Moss, Rankin, and Lewis, in "Types of British Vegetation" (1911) : 

"The different types of plant community on the same soil, namely, 'scrub' 
or bushland, and a corresponding grassland, or heathland, have no doubt 
originated mainly from the clearing of the woodland, and the pasturing of 
sheep and cattle. This prevents the generation of the woodland, and of most 
of the shrubs also, if the pasturage is sufficiently heavy and continuous, while 
it encourages the growth of grasses. Thus the plant formation determined by 
the particular soil, and once represented by woodland, shows a series of phases 
of degeneration or retrogression from the original woodland, brought about 
by the activity of man. The intimate relationship of the various phases is 
clearly seen in the associated plants. The woodland proper has of course a 
ground vegetation consisting of characteristic shade plants, but the open 
places, and the 'drives' and 'rides' of the woods, are occupied by many of the 
species found among the scrub and in the grassland, while those true woodland 
plants, which can endure exposure to bright light and the drier air outside the 
shelter of the trees, often persist among the grasses of the open. In some cases 
where grassland is not pastured, the shrubs and trees of the formation recol- 
onize the open land, and woodland is regenerated. Besides these degenera- 
tive processes, due to human interference, there are others due to 'natural' 
causes, which are for the most part little understood." (17) 

The degeneration of Quercetum roburis into subordinate or retrogressive 
associations of scrub and grassland is described (page 83), and the similar 
behavior of Quercetum sessiliflorae is discussed (page 130). An instructive 
discussion of reproduction in beechwood (168) lays bare the successional 
relations of the beech and ash, and at the same time serves to emphasize the 
fact that so-called degeneration is not a developmental but a destructive proc- 
ess due to man and animals. The last statement is also true of the behavior 
of heather moors, in connection with their repeated destruction by burning 
every few years (277) (plate 19 b.) 

The degeneration (retrogression) of moorland (280) obviously consists of 
two processes: "The earlier stages of the degeneration of a cotton-grass moor, 
in which the wetter Eriophorettim vaginati is replaced by the drier Vaccini- 
ehim myrtilli owing to gi*adual desiccation of the peat by improved drainage, 
are merely a normal stage of progressive development in which a hydrophytic 
sedge is replaced by a more mesophytie shrub. The sequence of life-forms and 
the reaction upon the water-content both prove that the movement is pro- 



154 DIRECTION OF DEVELOPMENT 

gressive and truly developmental, the drying due to erosion merely hastening 
the normal reaction. This is further proved by the statement that : 

"The five upland moor associations and their transitional forms described 
in the preceding pages form a series, Sphagnetum, Eriophoretum, Scirpetum, 
Vaccinietum, and Callunetum, showing a decreasing water content. . . . 
The desiccation of the peat may be continued till the moor formation is com- 
pletely destroyed. The first effect upon the vegetation is, as we have seen, 
the disappearance of the cotton grass and the occupation of the peat surface 
by the bilberry (Vaccinium myrtillus), the crowberry (Empetrum nigrum), 
and the cloudberry (Rubus chamaemorus) . As the process of denudation 
continues, this association gradually succumbs to changing conditions until the 
peat-hags become almost or quite destitute of plants. The peat, being no 
longer held together, is whirled about and washed away by every rainstorm or 
by the waters of melting snow. 

"In the end, the retrogressive changes result in the complete disappearance 
of the peat, and a new set of species begins to invade the now peatless surface." 

This is a convincing picture of the normal destructive action of erosion in 
producing new areas for succession, and the apparent retrogression or degen- 
eration of the moor thus resolves itself readily into the usual progressive move- 
ment of the dwarf-shrub stage, and the more or less rapid destruction of the 
latter, as well as the cotton-grass stage, by erosion. Destruction by erosion is 
also the explanation of the "phase of retrogression" found in the dune suc- 
cession when the "seaward face of the dunes is eaten away by the waves." 

Finally, Moss (1913) has extended the idea of retrogressive associations 
to include, it would seem, the larger number of communities in the Peak dis- 
trict of England. In discussing the degeneration of woodland (91), the 
author himself appears in doubt as to the natural occurrence of such a process. 
He says: 

"There can be no doubt that a certain amount of the degeneration of the 
woodland of this district has been brought about by the indiscriminate felling 
of trees, the absence of any definite system of replanting, and the grazing of 
quadrupeds. It is doubtful, however, if these causes are quite sufficient to 
account for so great a lowering of the upper limit of the forest as 250 feet 
(76m.) and for so general a phenomenon. ... It would appear to be true 
that, in districts which are capable on climatic and edaphic grounds of sup- 
porting woodland or true forest, the majority of examples of open scrub are 
to be regarded as degenerate woods and retrogressive associations. (94) . . . 
It would appear to be indubitable that woodland is frequently displaced by 
associations of scrub, grassland, heath, and moor. In all parts of the British 
Isles there has, within the historical period, been a pronounced diminution of 
the forest area, a diminution which, in my judgment, is in addition to and 
apart from any artificial deforestation or any change of climate. (96) . . . 
The conversion of woodland into scrub and of scrub into grassland, heath, 
or moor is seen not only on the Pennines, but in Wales, in the Lake District, 
and in Scotland. . . . Such successions are not exceptional in this country, 
but widespread and general; and whilst they are without doubt often due, 
in part, to artificial causes, it is at least conceivable that this is not always and 
wholly the case." (96) 

In an earlier paper (1907:44, 50), Moss states that ash-copse furnishes 
"the preliminary stages of a naturally forming ash -wood, or sometimes a 



REGRESSION 155 

vestige of a former extensive ash- wood," and apparently holds the opinion 
that "progressive" scrub is more frequent than "retrogressive" scrub. It is 
difficult to discover the reasons for his change of opinion. The absence of 
definite evidence of degeneration, and especially of retrogression, due to 
natural causes, militates strongly against the acceptance of his later view. The 
question of supposed degeneration is preeminently one in which quantita- 
tive methods through a number of years are indispensable. Quadrat and 
transect must be used to determine the precise changes of population and of 
dominance. Changes of habitat and the degree and direction of reactions 
must be determined by intensive methods of instrumentation, while the exact 
developmental sequence can only be ascertained by the minute comparative 
study of scar-rings and stump-rings, as well as that of soil-layers and relicts. 
Even the keenest general observations can not take the place of exact methods, 
which are alone capable of converting opinion into fact. 

Moss considers retrogression of the moor upon pages 166, 188, and 191. 
The point already made that the Vaccinietum myrtilli is always a stage in the 
normal progressive development is confirmed by the classification of moorland 
plant associations. (166) The discussion of retrogressive moors (188-189) 
adds further emphasis to the fact that retrogression is merely destruction due 
to denudation. 

"Whilst the peat of the closed association of Eriophorum vaginatnm is still 
increasing in thickness at a comparatively rapid rate, and that of the closed 
associations of heather and bilberry is also increasing though much more 
slowly, the peat on the most elevated portions of the moors is gradually being 
washed away. This process of physical denudation represents a stage through 
which, it would appear, all peat moors, if left to themselves, must eventually 
pass. Following Cajander [cf. Nilsson, p. 146], the associations thus formed 
are termed retrogressive ['regressive'] associations. 

"In the Peak District, the process of retrogression in the cottongrass 
moors is apparently initiated by the cutting back of streams at their sources. 
Every storm results in quantities of peat being carried away, in the stream 
winning its way further back into the peat, and in the channels becoming 
wider and deeper. Numerous tributary streams are also formed in the course 
of time, and eventually the network of peaty channels at the head coalesces 
with a similar system belonging to the stream which flows down the opposite 
hillside. The peat moor which was formerly the gathering ground of both 
rivers is divided up into detached masses of peat, locally known as peat hags 
and the final disappearance of even these is merely a matter of time. 

"It is obvious that this process results in a drying up of the peat of the 
original cottongrass moor ; and it is most interesting to trace a series of degra- 
dation changes of the now decaying peat moor. The first change of impor- 
tance of the vegetation appears to be the dying out of the more hydrophilous 
species, such as Eriophorum vaginatum and E. angusti folium, and the in- 
crease, on the summits of peaty 'islands' or 'peat-hags,' of plants, such as 
Vaccmium myrtillus and Empetrum nigrum, which can tolerate the newer and 
drier soil conditions. The composition of the upper layers of the peat of 
these retrogressive moors has, during the course of the present investigation, 
been carefully examined; and it has been found that the peat consists in its 
upper layers almost wholly of the remains of Eriophorum, The succession of 
cottongrass moor to the series of retrogressive moors here being described, is 
established beyond a doubt." 



156 DIRECTION OF DEVELOPMENT 

Hole's view. — Hole (1911 :13) defines progressive succession as follows : 

"A succession which thus proceeds from a xerophilous to a mesophilous and 
finally a hygrophilous type of vegetation, i. e., from a simple to what must be 
regarded as a more highly developed type, may be termed a progressive suc- 
cession. On the other hand, the reverse succession from a highly developed to 
a more simple type may be termed regressive. An example of such a succes- 
sion is seen when mesophilous forest is cleared, or more gradually destroyed 
by fire and grazing, the resulting erosion on steep slopes converting the area 
into a rocky hillside only capable of supporting the poorest and most xero- 
philous types of vegetation. Fire is a very potent factor in causing regressive 
successions, for it is not only capable of temporarily depriving the soil more or 
less completely of its covering of vegetation, but it also directly dries the soil 
and destroys the humus. Fire may in this way be responsible for the exist- 
ence of xerophilous grassland, or woodland, in localities which once supported 
mesophilous or possibly hygrophilous, vegetation. Grazing again, by destroy- 
ing the undergrowth and keeping a forest open, may so reduce the humus 
content of the soil as to render impossible the reproduction of the mesophilous 
species constituting the forest and may thus cause a regressive succession. 
Coppice fellings in the middle of a forest may similarly cause a regressive 
succession. 

"Finally there is a type of succession which we may distinguish as parallel 
succession. Types of both grassland and woodland are found in all kinds of 
habitats, ranging from the most xerophytic to the most hygrophytic, and it is 
of great importance to realize that for each type of grassland there is as a rule 
a corresponding type of woodland capable of thriving under similar conditions 
of environment, seeing that this has a direct bearing on the afforesting of 
grasslands. When a type of grassland, such as Munj savannah, is replaced 
by a parallel type of woodland, e. g., dry miscellaneous forest of Acacia, 
Dalbergia, and others, we may therefore regard it as a case of parallel suces- 
sion to distinguish it from the progressive and regressive changes considered 
above. Parallel changes can be effected more easily and rapidly than progres- 
sive changes, and with reference to such questions as the afforestation of 
grasslands and the extension of woodlands, parallel changes are as a rule of 
more importance." 

The author regards wet savannah, reed-swamp, and tropical evergreen forest 
as hygrophilous formations. Of these, the reed-swamp is usually regarded as 
hydrophytic, and, in extra-tropical regions at least, it never forms a final 
stage in succession. While Hole is evidently seeking the climatic climax in 
his definition of progressive succession, it seems doubtful that wet savannah 
and reed-swamp can be regarded as such. His view that progression passes 
through mesophytic stages to hygrophilous or hydrophytic ones is at variance 
with that of Cowles, in which mesophytic stages form the climax. While 
Cowles also regards movement from hydrophytic to mesophytic communities 
as progression, Hole does not consider this sequence at all. This conflict of 
opinion serves to emphasize the necessity of dealing with development alone, 
quite irrespective of the water character of the final stage. The author's state- 
ment that the progressive succession "proceeds from a simple to what must 
be regarded as a more highly developed type" is sound. But the types must 
be arranged upon the basis of life-form or phyad, and not upon habitat-forms 
determined by water. 



CONVERSION OF FOREST 157 

Hole's definition makes it clear why it was necessary for him to recognize a 
parallel succession. From the basic standpoint of development, parallel suc- 
cession is but the universal progression from lower to higher phyads charac- 
teristic of all seres. This is clear from the citation given above, but it is also 
shown by the following: 

"A very clear case where an area of recent alluvium has been first colonized 
by munj (Saccharum munja) in this way, but from which it has later been 
driven out again by the khair (Acacia catechii), has been seen by the writer in 
an area at the foot of the southern slopes of the Siwaliks near Mohan. In 
part of the area, munj is still dominant and vigorous, but young plants of 
khair are just appearing scattered here and there ; in other portions the khair 
are more numerous, larger and older, and many of the munj clumps between 
them can be seen dead and dying, while elsewhere a dense pure polewood of 
khair has become established under the shade of which can still be seen the 
decayed remains of the munj clumps which had first colonized the spot." 

Regression is defined as the "reverse succession from a highly developed to 
a more simple type." The illustrations given have been quoted above. It is 
again evident in these examples that the process is merely one of destruction 
by lumbering, fire, grazing, or erosion, with subsequent colonization by lower 
types. There is no succession, no development from forest to grassland, but 
a replacement of forest by grassland as a consequence of more or less complete 
destruction of the trees. As in all cases of supposed regression, the actual 
facts, in partially denuded areas especially, can be obtained only by quadrat 
and instrumental methods lasting through several years. 

Conversion of forests — The foregoing accounts seem to make it clear that 
nearly all cases of so-called retrogression or regression are not processes of 
development at all. They are really examples of the initiation of normal 
progressive development in consequence of destruction or denudation. Hence 
it is incorrect to speak of retrogressive succession or development, as well as of 
retrogressive formations or associations. The latter are merely those stages 
in which the production of a bare area occurs, with the concomitant origin of 
a new sere. Furthermore, the diverging views upon the subject indicate that 
the analysis has been superficial and extensive rather than intensive and 
developmental. 

There remain to be considered those cases in which a change from a higher 
climax community to a lower subclimax community actually occurs. Such 
are the actual and supposed cases of the. conversion of forest into scrub, 
heath, grassland, or swamp. The supposed examples of this change are 
numerous, but the process of conversion has been seen and studied in very few 
instances. This does not mean that the process may not be as universal as its 
advocates assume, but it does indicate that the final acceptance of this view 
must await intensive quantitative study of typical cases in each association. 
In this connection there are three distinct questions to be considered: (1) is 
it actually proven that the conversion of forest into heath or grassland does 
occur ; (2) can this change be produced by natural as well as artificial agencies ; 
(3) is it an actual successional development in a backward direction. 

Superficial evidence of the change of forest into grassland or heath is abund- 
ant in all countries where lumbering, grazing, and cultivation have been pur- 
sued for centuries. The rise of ecologv is so recent, however, and the number 



158 DIRECTION OP DEVELOPMENT 

of intensive quantitative studies for a period of years so few that there is 
hardly a ease in which conclusive proof is available. The well-nigh universal 
opinion of European workers in this matter merely constitutes an excellent 
working hypothesis, which can be accepted only after the most rigorous tests 
by exact ecology. The literature upon this subject is vast, but while much of 
it is suggestive, little is convincing. The dearth of conclusive evidence may 
best be indicated by the following statements from recent investigations. 
Graebner (1901:97) says: 

' ' In spite of the numerous moors with roots and upright stems that I have 
seen, for a long time I was unable to discover the swamping of a forest in the 
actual beginning of development. Finally, however, I had the opportunity 
of seeing two such moors in process of formation. One of these was found 
near Salm in western Prussia, the other at Kolbermoor in upper Bavaria." 

Status of forest in Britain. — The difficulties of determining the actual 
changes of woodland in the past may be gained from the statement of Moss, 
Rankin, and Tansley (1910:114) : 

"In a country like England, much of which has been cultivated and thickly 
populated for centuries, it may be asked, do there remain any natural wood- 
lands at all? Have not existing woods been so altered by planting and in 
other ways that they no longer represent the native plant communities, but 
are rather to be considered as mere congeries of indigenous and introduced 
species ? 

"It is undoubtedly true that there is little 'Urwald' or true virgin forest 
remaining in the country, though some of the woods, especially near the upper 
limit of woodland in the more mountainous regions, might make good their 
claim to this title. On the other hand, there are, of course, many plantations 
pure and simple which have been made on moorland, heath, grassland, or 
arable land, and which may of course consist of native or of exotic trees or of 
a mixture of the two. But between these two extremes, according to the con- 
clusions of all the members of the British Vegetation Committee who have 
given any special attention to this subject, come the great majority of the 
British woods; which are neither virgin forest, nor plantations de novo, but 
are the lineal descendants, so to speak, of primitive woods. Such semi-natural 
woods, though often more or less planted, retain the essential features of 
natural woods as opposed to plantations, and without any reasonable doubt 
are characterized by many of the species which inhabited them in their orig- 
inal or virgin condition. ' ' 

Moss (1913:111) concludes that: 

"Whilst opinions may differ as to whether or not the grassland just de- 
scribed is wholly or only in part due to man's interference, it appears to be 
generally accepted that such tracts were formerly clothed with forest; and 
Warming (1909 :326) even goes so far as to say that 'were the human race to 
die out,' the grasslands of the lowlands of northern Europe 'would once more 
be seized by forest, just as their soil was originally stolen from forest.' As 
regards the Nardus grassland of the hill slopes of this district, it seems incon- 
testable that it is an association which has, on the whole, resulted from the 
degeneration of oak and birch woods. The fundamental conditions of the 
habitat have been but slightly altered in the process ; and, therefore, the oak 
and birch woods, the Nardus grassland, and the various transitional stages of 
scrub are placed in one and the same plant formation. ' ' 



CONVERSION OP FOREST 159 

In America, the questions of the origin of the prairies, their derivation from 
forest, and their present tendency to become forest, have produced a copious 
literature, but the latter contains little or no conclusive evidence for one view 
or the other. 

Artificial conversion. — In spite of the almost total lack of direct proof, 
there is so much observational evidence of the artificial conversion of forest 
into scrub, heath, moor, or grassland as to create a strong presumption in 
favor of this view, and to furnish the most promising working hypotheses for 
intensive investigation. In the innumerable cases of the destruction of forest 
by cutting, grazing, fire, or cultivation, and the establishment of a subclimax, 
the feeling often amounts to positive conviction, which needs only experi- 
mental proof to be final. Indeed, many ecologists would doubtless regard the 
latter as altogether superfluous in most cases. In fact, one may well admit 
that all the evidence in our possession confirms the frequent change of forest 
to scrub or grassland where artificial agencies are at work. There is grave 
doubt when we come to consider the effect of natural causes in producing such 
changes. At present there is no incontestable proof of the conversion of 
forests by natural causes, except of course where effective changes in climate 
or physiography intervene. Graebner (1901:69, 97) has summarized the 
results of his own studies, as well as those of other investigators, and has fur- 
nished strong if not convincing evidence that forest may be replaced by heath 
or moor. It is significant, however, that in the various processes described 
by him, with one possible exception, the cutting of trees or an increase of 
surface water is required to initiate the changes which destroy the trees, and 
permit the entrance of Calluna or Sphagnum. In short, conversion is typic- 
ally the consequence of destruction and subsequent progressive development, 
often obscured by the fragmentary nature of the areas concerned. 

Graebner 's studies (1901): Conversion of forest to heath. — Graebner 's 
description of the process is so detailed and so convincing that a full account 
of it is given here. 

"Let us picture to ourselves the conversion to heath of a particular forest, 
such as may have obtained on the Luneberg Heath with the disappearance of 
the great forests. The calcareous pernutrient soil bears beech wood. The 
latter is completely removed as a consequence of the great demand for wood. 
While the ground remains bare and the forest slowly renews itself, the leach- 
ing-out of the nutrients in the soil proceeds more intensively, since the water 
formerly caught by leaves and mosses, and then evaporated, now soaks into 
the soil. Finally the forest again becomes closed, and then mature, and is 
again cut down. This may recur several times, during which the leaching-out 
of tbe upper layers in particular progresses steadily. With the decrease of 
nutrients in the upper layers, the growth of the herbs is made more and more 
difficult, until finally these die out, since their roots are unable to reach into 
the deeper unleached layers of soil. As a consequence, all herbs which demand 
relatively large amounts of nutrients are excluded. The competition of plants 
with low requirements and slow growth disappears, and leaves the field to 
heath plants. 

"At first the heath plants colonize but sparsely beneath the trees. In such 
a forest, one sees a few heath plants here and there, especially Call una, which 
have however a suppressed look because of the deep shade still found in most 
places. The growth of tree seedlings in the poor sandy soil becomes greatly 



160 DIRECTION OP DEVELOPMENT 

handicapped. Seeds of the beech germinate normally, but in the first few 
years the seedlings show only a weak growth, especially of the aerial parts, as 
a consequence of the poor soil. Ultimately, the growing roots reach the lower 
richer soil layers, and the young saplings then begin to stretch upwards. 
They develop dense thickets in the gaps due to fallen trees, and thus hinder 
the further development of heath plants. Such forests have mostly a very 
poor flora, since forest plants lack for nutrients, and heath plants are sup- 
pressed by the dense shade. In such places, a complete conversion to heath 
could occur not at all or only after a long period, since the layer of leached 
soil must attain such a thickness that the seedlings disappear before their roots 
reach the deeper nutrient layer. In this event, it is more probable that the 
beech will be replaced by a tree with lower requirements, such as the pine, 
before this finally yields to the heath. 

"The formation of 'ortstein' hinders the reproduction of the forest, as soon 
as the leached layer becomes so thick that frost can not penetrate to its lower 
limit. At this level, the precipitation of dissolved humus compounds, leached 
out of the soil above, cements the sand into a humus sandstone. In the heath 
regions, the latter is laid down for miles as a pure uninterrupted layer at a 
depth of one foot as a rule. As soon as the 'ortstein' has attained a certain 
thickness and density, it can not be pierced by plant roots. The latter can 
penetrate only in small gaps which maintain themselves here and there in the 
layer. The upper leached layer is thus almost completely separated from the 
lower nutrient layer. The variations in water content are marked and can 
no longer be affected by capillarity. As soon as the 'ortstein' begins to develop 
in the forest, the latter takes on a different look. The roots of beech seedlings 
and of young plants of the undergrowth can not penetrate the 'ortstein' and 
reach the lower soil layer. They languish for a time, and then perish as a 
consequence of lack of nutrients and water, or of the winter killing of the 
unripened wood. Undergrowth and reproduction begin to disappear. The 
gaps produced by the fall of old mature trees are not filled with new growth, 
and thus afford favorable conditions for heath vegetation. The forest becomes 
more and more open through the death of old trees, the heath develops cor- 
respondingly, and soon becomes dominant. After a few decades only isolated 
trees remain upon the bare field. Elsewhere all is heath. 

"Such are the general features of the process by which the vast stretches of 
heath have arisen from forest. To-day we have all stages of the development 
of deciduous wood of beech and oak to typical heath, especially in the eastern 
transition regions. Conversion to heath is naturally hastened by the clearing 
and utilization of the forest, though it must occur even without this, through 
the operation of climatic factors upon sandy soil. 

"The conversion of pine forest into heath is quite similar to that of beech 
forest, though the lower requirements of the pine enable its seedlings to thrive 
better in the leached soil. The leaching-out process also proceeds more rapidly 
owing to the lower nutrient content, but the development of 'ortstein' is less 
marked. This is due to the fact that the looser canopy of the pine forest, as 
well as the sparser undergrowth, permits the sun and the wind to hasten 
decomposition, in relation to humus production. On the protected floor of 
the beech forest, on the contrary, the formation of humus is more marked than 
decomposition, and there is in consequence a larger supply of humus com- 
pounds for precipitation as the cement of 'ortstein.' 

"A further method of heath formation is considered by Grebe (1896). In 
this, the decomposition of the fallen needles or leaves takes place so slowly 
in dense shady woods, especially of fir and in moist climates, that by far the 



CONVERSION OF FOREST 161 

bulk of the material is converted into humus, which gradually compacts itself 
into a firm layer. Heath formation on such a soil is interesting for the reason 
that it may occur without the leaching of the upper layer, and indeed may be 
found on heavy loam or clay. Grebe describes the action of the raw humus 
upon vegetation as follows: ' (1) The raw humus cuts off the lower soil almost 
completely from its air supply. (2) It hinders the circulation of water in the, 
soil. It prevents the evaporation of superfluous moisture in winter and spring, 
and in summer it hinders the penetration of light rains and of dew. (3) It is 
probable that the soil beneath the layer of heath-felt passes out of the stage 
of oxidation into that of stagnation and reduction. (4) The upper soil layer 
is relatively poor in dissolved mineral salts, the middle and lower relatively 
rich. (5) While the raw humus of the heath is as rich as the humus of the 
beech woods and pine woods, it is so firmly combined as a consequence of its 
peaty nature that it can not be used by the trees. ' 

"Grebe has been correct in his assumption that the aeration of the soil is 
almost completely prevented by the raw humus. According to my opinion, 
this factor suffices almost entirely alone to make the proper growth of trees 
impossible and to call forth sickness, stunting, or death according to the inten- 
sity of its action." 

Conversion of forest into moor. — "It is generally recognized that the 
heath moor differs from the meadow moor in that it is not level but convex. It 
grows upward not only in the middle, but also, even though slowly, at the 
margin. Now if such a moor arises in a shallow depression, it slowly pushes 
its edges up the slopes. Thus it finally reaches a gap in the surrounding hills, 
and it then extends a tongue through the gap into further levels. Thus it 
comes about that a lively movement of water is noticed, when the tongue of the 
moor lies upon sloping ground. Since the tongue lies lower than the surface 
of the moor and the Sphagnum holds the water so firmly that the surplus can 
soak into the soil but slowly, the tongue is constantly dripping with water, 
and in most cases a quantity of water flows away from it, as at Kolbermoor. 
If the soil of the slope and adjacent lower areas is not especially pernutrient 
at its surface, the formation of heath moor proceeds rapidly. The cushions of 
Sphagnum spread more and more widely till they reach the bottom of the low 
area always fed with water from above. The bottom once reached, the con- 
stant flow furnishes abundant water for further development, unless, as is 
frequently the case, a colony of Sphagnum has already occupied the bottom as 
a consequence of the accumulating water, in which event the two masses unite. 
Whenever the hollow or the slope and gap are covered with forest, the soil is 
converted into swamp by the Sphagnum and the air is driven out as a result. 
The physiological effect upon the growth of trees is the same as in the forma- 
tion of raw humus upon the forest floor. It is a peculiarly desolate picture 
that is formed by the countless dead standing trunks in a young moor. One 
trunk after another falls, and soon they are all buried in the moor, and noth- 
ing visible remains to remind one of the former forest. 

"In order to exhibit the entire process of the swamping of a forest, I have 
purposely chosen cases in which the moor must pass over a small elevation, 
since the important events in the water movement are much clearer than in 
the common instances. For the most part, the formation of heath moor upon 
meadow moor, or also in lowland forest, takes place completely on the level, 
and in the following manner. The lowlands have become filled with meadow 
moors [swamps], as a result of the forlanding of ponds and lakes, and the 
consequent development of swamps. The ground level of the swamp slowly 
grows upward because of the annual increment of plant remains, but only to 

12 



162 DIRECTION OF DEVELOPMENT 

the point where plants are still able to obtain the necessary water supply. 
Wherever the swamp is built above the level of the ground water, trees, espe- 
cially alders and oaks, enter and form forests. Often, however, the swamps 
remain treeless. The swamp peat has the peculiarity that is conducts water 
very poorly, in contrast to heath moor peat, which has a marked conductive 
power. As a result, the swamp plants disappear as soon as their roots are no 
longer able to penetrate into the subsoil, and at the outset the flat-rooted 
plants disappear. There remains a community of tall perennials, mostly 
grasses. 

"This is the point at which the change to heath moor begins. Sphagnum 
colonizes the lower moister places, and in similar fashion as upon the moist 
sandy soils, the cushions run together and first fill the hollows and ditches in 
the swamp. As soon as the Sphagnum has reached a certain extent, and has 
filled the bottom of a ditch or hollow, other conditions of moisture begin to 
appear. While previously a single dry sunny day sufficed to dry out and heat 
up the black surface of the moor, the Sphagnum cushions now hold the water 
with great tenacity. Even after a long dry period, the moss turf is still 
moderately moist within, while elsewhere it is dried out. In early stages, the 
Sphagnum occurs only in ditches and hollows, which soon become completely 
filled. When the moss layer has reached a certain thickness, it forms a great 
reservoir of water, and the upward growth of the moss constantly increases. 
It then spreads laterally over the level surface of the swamp, always carrying 
larger quantities of water, which is unable to sink away because of the marked 
imperviousness of the swamp peat which underlies the heath peat. After a 
time, the various Sphagnum masses grow together and close over the swamp. 

"The primary requisite for such a moor, in so far as an actual inflow of 
water is concerned, is that the annual precipitation should be greater than the 
loss of water by evaporation and percolation. Here must be noted the fact 
that the marked affinity of Sphagnum and heath peat for water, as well as 
the very impervious nature of peat when it is saturated, produces very dif- 
ferent water relations than those which prevail in the swamp. The depend- 
ence of such moors upon the rainfall of a region also explains the great fre- 
quency of heath moors in the great heath regions, and their infrequence or 
absence in dry climates (98-100). 

"In cases where a forest has developed upon a meadow moor before the 
beginning of a moss moor, the development of a heath moor takes place more 
rapidly. This is obviously due to the protection which the trees afford the 
Sphagnum against sudden drouth. In such forests one almost never finds 
small scattered cushions, but nearly always great masses or connected mats. 
In an open swamp in which heath moor is beginning its development, one finds 
on the contrary that the small dense moss cushions, located in small depres- 
sions under the scanty shade of grass tufts, have their stems much compacted 
and often show a red color. This indicates that the mosses live there only on 
sufferance, and that they scarcely secure enough water to last through a dry 
period. 

"The second method of origin of heath moor upon bare soil is that found 
in some meadow moors. One very often finds in moors of great depth that 
there is at bottom a more or less thick layer of black swamp peat, which 
passes through a definite zone, often with tree trunks, into the heath peat 
above. Not rarely, especially in northwestern Germany, the heath peat shows 
an upper and lower layer. The development of heath moor in swamp in 
such cases must have been due to a change of water relations, as a conse- 
quence of which the swamp was flooded with enutrient water. Such instances 



CONVERSION OF FOREST 163 

must, however, occur but rarely. In the majority of cases, heath moor arises 
in a swamp very much as it does in forest. It is best in consequence not to 
separate the consideration of the two, especially since the sections of moors 
show that a layer of tree roots is very often found at the edge of the swamp 
or heath moor layers. Such a moor was forested before it became covered 
with heath moor." (96) 

Causes of conversion. — Graebner has described six processes by which 
forest or swamp is converted into heath or heath-moor. In the first, forest is 
changed into heath as a result of the removal of the trees in whole or in part, 
with consequent leaching of the upper layer and the formation of "ortstein." 
The need for destroying the reaction control of the trees, i. e., their shade, is 
shown by his statement that Calluna has a suppressed appearance because of 
the deep shade still found in many places. The artificial destruction of the 
forest seems requisite. Graebner says that ' ' the conversion to heath is natur- 
ally hastened by the cutting and utilization of forest, though it must occur 
even without this, through the operation of climatic factors upon soil. ' ' Our 
present knowledge seems quite inadequate to confirm this statement. On 
theoretical grounds, such conversion would seem quite impossible without the 
contributing action of climatic variation, since a climate constantly like that 
under which conversion occurs would have prevented the development of the 
original forest. The work of Douglass (1909, 1914), Humphreys (1913), and 
Huntington (1914), seems to indicate clearly that so-called changes of climate 
are but the persistence for a time of variations such as occur from year to 
year. It seems probable that the conversion of forest into heath as a result of 
the formation of raw humus is a consequence of such climatic variations, and 
that it is further aided by the influence of man and domesticated animals. 
Graebner himself nowhere considers this matter of climatic oscillations, since 
he is concerned primarily with the detailed changes in the soil. It is, however, 
of the most vital importance in determining the real nature of secondary 
development, since regression can be said to occur only when the reactions 
of the undisturbed vegetation produce an actual backward sequence of com- 
munities. 

Of the four ways by which heath moor may arise from an existing swamp 
or forest, one, the flooding of a swamp by enutrient water, is obviously a 
matter of destruction and denudation. A careful analysis of the other cases 
likewise shows that the process is here one of flooding and destruction. The 
essential fact that the change is due to flooding is obscured by the intimate 
interrelation between Sphagnum and water, and by the appearance of Sphag- 
num in many separate spots. Ecologically, the water-soaked moss is the equiv- 
alent of the direct flooding of an area by so much water, except that the 
Sphagnum water has a much more marked effect, since most of it can not drain 
off, and since the amount constantly increases. The Sphagnum is really a 
pioneer in a new if minute water area, and differs only in degree from the 
thalli of algal pioneers, such as Nostoc, which also absorb and retain water 
tenaciously. Graebner 's statements also support this view, for he says: 

"This is the point at which the change to heath moor begins. Sphagnum 
colonizes the lower moister places and, in similar fashion as upon the moist 
sandy soils, the cushions run together and first fill the hollows and ditches in 
the swamp. The Sphagnum cushions now hold the water with great tenacity. 



164 DIRECTION OF DEVELOPMENT 

When the moss layer has reached a certain thickness, it forms a great reservoir 
of water. It then spreads laterally over the level surface of the swamp, 
always carrying larger quantities of water. 

"Since the tongue (of moss-turf) lies lower than the surface of the moor 
and the Sphagnum holds the water so firmly that the surplus can soak into the 
soil but slowly, the tongue is constantly dripping with water, and in most 
cases a quantity of water flows away from it. The bottom once reached, the 
constant flow furnishes abundant water for further development, unless, as is 
frequently the case, a colony of Sphagnum has already occupied the bottom, as 
a consequence of the accumulating water. The soil is converted into swamp 
by the Sphagnum and the air is driven out as a result. ' ' 

These are the precise consequences of ordinary flooding by water, and like- 
wise lead to destruction of the grassland or forest. 

To sum up, while there is abundant evidence that forest is being changed 
into scrub, heath, or grassland as a result of the action of artificial causes, 
there is no convincing proof that such conversion can occur under existing 
natural conditions. In all cases cited, disturbance by man is either a certain 
or probable factor, or the destruction has been a consequence of topographic 
or climatic changes. In no case is there clear proof, as a result of continued 
quantitative investigation, that a forest produces changes inimical to its exist- 
ence and favorable to a lower type of vegetation. 

Possibility of backward development. — In all cases of the change of forest 
to scrub or grassland, even if they be admitted to result from artificial disturb- 
ance in some degree, it would seem at first thought that the process is actually 
a backward development, i. e., retrogression. In all the instances cited above, 
however, as well as in all of those so far encountered, the only development is 
that of a new community on ground left partially or completely bare by the 
forest. There is no difficulty at all in recognizing this when the ground is 
entirely denuded by a fire, and but little when the trees are completely de- 
stroyed by clean cutting. Similarly, when areas of some extent are cleared 
in the forest, it is sufficiently obvious that the communities which appear in 
the clearing are the result of the destruction of the former dominants, and of 
consequent invasion into a sunny though localized habitat. When, however, 
such areas are no larger than the space made by the fall or removal of a single 
tree, the situation is more complex. A comparison of a number of such small 
areas, alternating with each other as well as with clearings of various size, 
would give the impression of an actual retrogression. This would be due to 
the amount and kind of invasion in denuded areas of widely differing extent, 
and the consequent persistence or adaptation of the original undergrowth in 
varying degrees. Indeed, a general comparison of such areas can not be 
expected to yield the real facts. It is only by the exact study of each cleared 
area, large or small, that the true nature of the process stands revealed. Such 
an investigation will invariably show that, no matter how small an area may 
be, it has a progressive development all its own, but in every respect in essen- 
tial harmony with the development in a large clearing of the same forest, or 
in an extensive denuded area of the same type. In every case it is found that 
there is no backward development, but merely a fictitious appearance of it due 
to destruction of the dominants in large or small degree, and the immediate 
invasion of species best adapted to the conditions of the new area. The care- 



CLEMENTS 






A. Dost ruction of woodland of Pimts torreycma by fire and erosion, and replacement 

by chaparral, Del Mar, California. 

B. Eoot-sprouting from the base of burned chaparral dominants, Qm reus. Arcto- 

staphylus, etc., Mount Tamalpais, California. 

C. Destruction of mixed prairie and invasion by woodland and scrub, bad lauds. 

Crawford, Nebraska. 



REGENERATION 165 

ful scrutiny and investigation of thousands of cases of local or minute denuda- 
tion in various associations permit of no other conclusion (plate 20). 

An actual retrogressive development, a regressive succession, would neces- 
sarily move backward through the same communities, represented by the same 
phyads and reactions, as those through which the sere progressed. No one 
has yet furnished the slightest evidence of such a development, and according 
to the views set forth here, such a movement is absolutely impossible. It can 
no more take place than an adult plant can be devolved again into a seedling 
or a seed. The adult plant may be destroyed, and the seedling may take its 
place. In like manner, a climax or subclimax community may be destroyed, 
and an earlier associes develop in its stead. But a backward development is 
as impossible in the one case as in the other. Destruction and reproduction 
are the only possible processes. Even if one were to attempt to remove all 
the individuals of each community in the reverse order of sequence, a true 
retrogression comparable with the normal progression would still be impos- 
sible, without at the same time destroying the reactions pari passu and estab- 
lishing the dominants of the next earlier associes. 

That the development after lumbering is the normal progression due to 
partial denudation is shown by the observation of Adamovic (1899:144) in 
the Balkans. He summarizes the secondary succession as follows: The first 
stage occurs a few months after cutting. It is characterized by the disappear- 
ance of shade plants, Oxalis, Actaea, Daphne, Dentaria, etc., and the increase 
of the species found at the margin of the wood, such as Gentiana, Salvia, 
Knautia, Digitalis, Senecio, etc. The second stage is marked after a few years 
by the development of a scrub of Corylus, Crataegus, Lonicera, etc., with an 
undergrowth of Poa nemoralis, Rhinanthus, Pyrethrum, etc. The third stage 
appears after 8 to 10 years, and is characterized by a young growth of Fagvs, 
Betula, Acer, and So7-bus, with a height of 5 to 6 feet. 

Degeneration. — It follows from the above that communities do not degen- 
erate. They can only be destroyed with greater or less rapidity over larger 
or smaller areas. As indicated above, there can be no thought of degeneration 
when a forest is completely removed by fire, flood, or ax. This is too obviously 
the normal process of denudation and secondary development. But when the 
destruction is piecemeal, or when it acts through many years, the superficial 
appearance of the community with its areas of normal structure side by side 
with bits of earlier stages and actual bare spots seems to warrant the con- 
elusion that the community is degenerating. Sueh a condition is strikingly 
shown in the moors of the Pennines. The independent study of each area 
shows, however, that this is only a complex of moor communities in varying 
stages of progressive development, alternating with areas exhibiting denuda- 
tion in different degrees. All so-called degenerating associations are to be 
explained in the same way (plate 19a). 

Regeneration. — While the term "degeneration" is both incorrect and mis- 
leading, no such objection can be brought against "regeneration" or "rejuve- 
nation." This follows quite naturally from the fact that succession is always 
progressive, but never retrogressive. A climax formation reproduces itself in 
whole or in part, depending upon the degree of denudation. When the latter 
results in the production of a secondary area, the reproduction is essentially 
that which occurs in the case of a plant regenerated from a leaf, and the 



166 DIRECTION OF DEVELOPMENT 

term "regeneration" might be applied to all secondary succession. Rejuve- 
nation is essentially synonymous, though it would seem to include primary 
successions rather more readily. The only objection to be urged against them 
is that their use tends to suggest that some process other than normal succes- 
sion is concerned. Used as synonyms of succession, they are unobjectionable, 
though as a consequence they are also of little value. 

Correlation of progressive developments. — While all successional develop- 
ment is progressive, the concrete seres of every climax formation may bear a 
direct relation to the whole course of development. This is fundamentally true 
of the seres which arise in primary and secondary bare areas and hence are 
distinguished as primary and secondary seres. The one recapitulates the entire 
succession, the other repeats only more or less of its later sequence. Seres, 
moreover, show an essential difference with respect to the direction of reaction, 
depending upon the nature of the extreme conditions in which they arise. 
Primary seres may arise on rock or in water, or they may develop on new soil, 
such as that of dunes or bad lands. "While secondary areas do not depart so 
widely from the climatic mean, they may also be xerophytic or hydrophytic. 
Though often mesophytic, they are always drier or wetter than the climax area. 

The basic developmental relation of every sere is indicated by the terms 
prisere and subsere. The one is a concrete example of primary succession, 
the other of secondary succession. Since they mark a fundamental distinction 
in the development of a climax formation, their further treatment is deferred 
to the chapter upon classification. 

As water-content is the controlling factor in all succession, either directly or 
indirectly, it furnishes the best basis for indicating the direction of movement. 
This arises from the fact that it represents the primary interaction of habitat 
and community in the course of development. In the origin of every sere, 
the amount of water is the critical factor, and the rate and direction of devel- 
opment will be recorded more or less clearly in its increase or decrease. There 
are in consequence three possible bases for distinguishing direction in terms of 
water-content. These are (1) the actual direction of movement itself, (2) the 
initial condition, (3) the final condition. It is of interest to note that all of 
these have been used. Clements (1904:124; 1905:257) made use of the actual 
successional change in water-content, as well as the final term : 

"The direction of the movement of a succession is the immediate result of 
its reaction. From the fundamental nature of vegetation, it must be expressed 
in terms of water-content. The reaction is often so great that the habitat 
undergoes a profound change in the course of succession, changing from 
hydrophytic to "mesophytic or xerophytic, or the reverse. This is character- 
istic of newly formed or exposed soils. Such successions are xerotropic, meso- 
tropic, or hydrotropic, according to the ultimate condition of the habitat. 
When the reaction is less marked, the type of habitat does not change mate- 
rially, and the successions are xerostatic, mesostatic, or hydrostatic, depending 
upon the water-content. Such conditions obtain for the most part only in 
denuded habitats." 

Cooper (1912 :198) has made the initial conditions the basis of classification : 

"The plant successions leading up to the establishment of the climax forest 
are conveniently classified in two groups : the xerarch successions, having 
their origin in xerophytic habitats ; and the hydrarch successions, originating 
in hydrophytic habitats." 



CONVERGENCE 167 

Cowles (1901) and Hole (1911), as already mentioned, have used the final 
condition as a basis for distinction. While both use the terms progressive and 
retrogressive or regressive, Cowles regards all development toward a meso- 
phytic condition as progressive, while Hole employs this term for movement 
toward a hygrophilous or hydrophytic climax. The disadvantages of the use 
of the terms progressive and retrogressive have already been discussed. 

The emphasis here laid upon the climax formation as an organic unit with a 
characteristic development would seem to make terms based upon the course 
of the reaction and the final condition unnecessary. In all cases the progres- 
sive development leads to the highest life-form possible, and the tendency of 
the reaction upon water-content is usually toward a mesophytic mean. Excep- 
tions occur only in dry regions or in moist tropical ones. Hence the nature of 
the climax formation indicates the direction of movement, and the terms 
mesotropic, xerotropic, mesostatic, etc., hardly seem necessary at present. 
To one who does not know the general conditions of a climax formation, they 
are useful, but there is little need for them until more hydrotropic and xero- 
tropic seres are known. This does not seem true of the terms hydrarch and 
xerarch since they indicate the extreme condition in which the seres originate, 
though they also indicate by inference the general course of development. 
Since it is the kind of initial bare area which gives character to all the earlier 
stages of a sere, hydrarch and xerarch are now of much value in introducing a 
basic distinction into both primary and secondary succession. They suggest 
the normal movement toward the mesophytic mean, but are hardly applicable 
to seres which are xerotropic or hydrotropic. As a consequence, it may prove 
desirable to employ the latter terms for the sake of completeness, even in the 
present state of our knowledge. 

Convergence. — It is obvious that all the seres of a climax formation con- 
verge to the final community. No matter how widely different they may be in 
the pioneer stages, their development is marked by a steady approach to the 
highest type of pliyad possible in the climatic habitat and to a corresponding 
water-content. The pioneer lichens of a rocky ledge and the pioneer alga? of 
a pool both initiate seres, which are characterized by increasingly higher 
phyads and more and more medium water-contents, until both terminate in 
the climatic climax of both vegetation and water, as, for example, in the 
grassland of the Great Plains. 

This fundamental convergence to a climax is developmental, and not indi- 
vidual or local. Each sere in itself is a unit development which moves in the 
inevitable direction from bare area to climax. Convergence is visible only in 
a survey of the succession in the climax association as a whole. The actual 
situation suggests an imaginary developmental cone formed by lines converg- 
ing from a broad base of various primary and secondary areas through grass- 
land and scrub to the final climax forest. Thus, while the development in 
every bare area, e. g., rock-ledge, pond, burn, fallow field, etc., is a unit com- 
prising the whole range from the initial extreme to the climax, the seres taken 
collectively are identical in one or more of the final stages. Convergence may 
be upon practically any stage in the succession, but it is usually upon a sub- 
climax stage of grassland or scrub in the case of forest, for example. In addi- 
tion there is often an earlier convergence of primary seres, especially upon 
some medial stage. 



168 DIRECTION OF DEVELOPMENT 

Cooper (1912:198) has used the term "subsuccession" for the seres which 
begin on rock-surfaces, in crevices and in rock-pools, and terminate in the 
formation of a heath-mat. Thus, he distinguishes a rock-surface subsuc- 
cession, a crevice subsuccession, and a rock-pool subsuccession of the rock- 
shore succession. However, he does not apply the term to seres which con- 
verge later in the development. The phenomenon is the same whether it 
appears early or late in succession. It is here proposed to apply the term 
adsere (ad-, to, implying convergence) to that portion of a sere which precedes 
its convergence into another at any time before the climax stage. While it 
is possible to distinguish adseres with respect to convergence in the initial, 
medial, or subclimax stages, at present it does not seem wise to do so. Like- 
wise, a developmental line formed by the convergence of two or more adseres 
may itself converge and become an adsere (fig. 5). The use of subsucession 
in this connection seems undesirable because of the fundamental distinction 
already drawn between succession and sere. 

Normal movement. — It is probable that the large majority of all the seres 
of a climax association pass through their development in the normal manner. 
All the stages are represented ; they follow each other in the usual sequence 
and progress at about the same rate. But the normal course of development 
may be disturbed or changed in various ways. Frequently the modification 
is merely one of rate, and succession takes place in the usual way, but at a 
faster or slower pace. Distinctions upon the rate of movement can hardly be 
made at present, as our exact knowledge of succession is still small. There 
are many seres, however, in which it has been shown that artificial or topo- 
graphic changes have hastened or retarded the normal course. This disturb- 
ance may be so great that the sere is held for a long time in some associes, 
which in consequence takes on the appearance of a climax. Or, as a result 
of the absence of the usual climax species, the subfinal stage may become the 
actual climax. 

Apart from such modifications as these in which the sequence is not affected, 
there are those in which stages are dropped out or interpolated, or in which 
there is a deflection of the course of movement. The failure of a particular 
stage to develop is a frequent occurrence in seres with many stages, particu- 
larly when the reaction of each is not especially marked. In such cases, the 
sequence is determined largely by migration, and the relative abundance and 
nearness of the dominants of two or three associes is decisive. On the other 
hand, the interpolation of an unrelated stage occurs but rarely, since it can 
take place only when a new dominant enters the region, as in the case of 
weeds. A complete change in the course of development apparently can 
result only from a change of climate. Such changes necessarily affect the 
climax vegetation, and hence are considered in later chapters. 

These various modifications have previously been recognized and distin- 
guished by terms (Clements, 1904:107, 122; 1905:240, 254). Normal suc- 
cession begins with nudation, and passes through the regular sequence to the 
climax association. Anomalous succession occurs when the sequence is 
destroyed by addition or subtraction, or when the succession is deflected. 
Imperfect succession results when one or more of the ordinary stages is omitted 
anywhere in the course and a later stage appears before its turn. It will 
occur at any time when a serai area is so surrounded by dense vegetation that 



DIVERGENCE 169 

the communities which furnish the next invaders are unable to do so, or when 
the abundance and mobility of certain species enable them to take possession 
before their proper turn, and to the exclusion of the regular stage. When a 
stage foreign to the succession is inserted, replacing a normal consocies or 
slipping in between two such, the development may be called interpolated 
succession. 

Divergence. — Graphic representations of the development to a climax 
often show divergence as well as convergence. This is frequently due to the 
ability of a particular consocies to develop in one serai area but not in another. 
The corresponding diagram often shows a divergence in such cases when none 
actually occurs. Usually, however, apparent divergence arises from connect- 
ing the development of secondary seres with preceding primary ones, or from 
the presence of two or more nearly equivalent communities, such as Scirpus 
caespitosus and Eriophorum, or the alternation of consocies, such as Typha, 
Scirpus, and Phragmites, which may occur separately or variously grouped. 
"Within the same climax formation actual divergence is rare if not impossible. 
It can occur for a time when a foreign dominant is interpolated and it would 
take place if climatic changes were to affect one part of a great climax area 
and not another. On the other hand, while the initial stages on rock, in 
water, and on dune-sand are identical or similar throughout the northern 
hemisphere, the final climaxes differ widely. This is a natural consequence of 
the fact that relatively few species can grow in extreme conditions, and that 
such species are usually able to migrate widely. As a consequence, a few 
communities form the pioneer and initial stages of the development of a 
large number of climax associations. The result is that the corresponding 
seres diverge just as soon as the initial extremes become modified to the point 
where the effect of the various climates begins to be felt. Such divergence, 
however, is a feature only in the composite picture of vegetational develop- 
ment in North America and Eurasia. In the case of each climax formation 
it is absent. 



IX. CLASSIFICATION OF SERES. 

Historical. — While the division of successions into progressive and regres- 
sive by Nilsson (1899) may be regarded as an early attempt at classification, 
the first system of classification for successions was proposed by Clements 
(1904:107, 138; 1905:241). Cowles (1901:86) had already advanced his 
physiographic grouping of the plant societies in the region of Chicago. While 
this necessarily threw successions into topographic groups, his whole intent 
was to classify plant societies or associations upon a genetic and dynamic basis 
(I. c, 178), and hence he did not consider the classification of successions. 
Later (1911:161), he discusses the causes of vegetative cycles, and proposes a 
classification upon this basis. These two systems are the only ones yet sug- 
gested, and as they have much in common it is desirable to consider them in 
detail before taking up the system proposed here. 

Clements 's system. — This was based primarily upon development, with 
especial reference to reaction, and secondarily upon initial causes, in which 
topographic causes were recognized as paramount. The division into normal 
and anomalous successions, and the subdivision of normal successions into 
primary and secondary were both based upon development. The subdivisions 
of primary successions were all grounded upon topographic processes, and 
those of secondary successions upon topographic and biotic agencies, while 
anomalous successions were primarily due to climatic changes. The essential 
features of the classification are indicated by the following outline: 

I. Normal successions. I. Normal successions — Continued. 

1. Primary successions. 2. Secondary successions — Continued. 

(1) By elevation. (4) In landslips. 

(2) By volcanic action. (5) In drained and dried-out soils. 

(3) In residuary soils. (6) By animal agencies. 

(4) In colluvial soils. (7) By human agency. 

(5) In alluvial soils. a. Burns. 

(6) In aeolian soils. b. Lumbering. 

(7) In glacial soils. c. Cultivation. 

2. Secondary successions. d. Drainage. 

(1) In eroded soils. e. Irrigation. 

(2) In flooded soils. II. Anomalous successions. 

(3) By subsidence. 

With reference to the initial physical or biological cause, a normal succes- 
sion was defined as one which begins with a bare area and ends in a climax, 
while anomalous succession was defined as that in which an ultimate stage 
of a normal succession is replaced by another stage, or in which the direc- 
tion of movement is radically changed. The former was stated to be of 
universal occurrence and recurrence ; the latter operates upon relatively few 
ultimate formations. Anomalous successions were regarded as the usual result 
of a slow backward-and-forward swing of climatic conditions. Primary suc- 
cessions were defined as those that arise on newly formed soils, or upon sur- 
faces exposed for the first time. Such areas have in consequence never borne 
vegetation before. They present extreme conditions for ecesis, and possess few 
or no dormant disseminules. Accordingly, primary successions take place 
slowly and exhibit many stages. Secondary successions arise on denuded 

170 



PRIMARY AND SECONDARY SUCCESSION 171 

soils, except in cases of excessive erosion. Denuded soils as a rule offer 
optimum conditions for ecesis, as a result of the action of the previous suc- 
cession; dormant seeds and propagules are abundant, and the revegetation of 
such habitats takes place rapidly and shows relatively few stages. The great 
majority of secondary successions owe their origin to fire, floods, animals, or 
the activities of man. They agree in occurring upon soils of relatively medium 
water-content, which contain considerable organic matter and a large number 
of dormant migrants. 

Successions were also classified as imperfect, continuous, intermittent, 
abrupt, and interpolated upon the basis of the nature of development. Initial 
causes were classified as (1) weathering, (2) erosion, (3) elevation, (4) sub- 
sidence, (5) climatic changes, (6) artificial changes. The reactions of succes- 
sion were summarized as (1) by preventing weathering; (2) by binding 
aeolian soils; (3) by reducing run-off and preventing erosion; (4) by filling 
with silt or plant remains; (5) by enriching the soil; (6) by exhausting the 
soil; (7) by accumulating humus ; (8) by modifying atmospheric factors. It 
was further stated that a natural classification of successions will divide them 
first of all into normal and anomalous. The former fall into two classes. 
primary and secondary, and these are subdivided into a number of groups, 
based upon the cause which initiates the succession. 

Normal and anomalous succession. — The persistent study of successional 
development for the decade since the preceding views were enunciated seems 
to have confirmed and emphasized the distinction between normal and anoma- 
lous succession. Normal succession is unit succession, that is, the development 
from an initial bare area to a climax. It is represented by the sere, with its 
distinctions of prisere and subsere. Anomalous succession may be termed 
compound succession, i. e., that in which similar or related seres are combined 
into a cosere as a consequence of climatic action. It is represented by the 
cosere and clisere, and in its major expression by the great successions of 
geological eras, the eoseres. Since climate rarely if ever produces a denuded 
area of any extent, the earlier distinction of normal and anomalous succes- 
sions conforms closely to the present division into seres and cliseres. The 
former are essentially topographic or biotic as to cause, the latter are funda- 
mentally climatic. Cowles (1911:170) has also recognized the validity of 
this distinction in contrasting climatic or regional successions with topo- 
graphic and biotic ones. 

Primary and secondary succession. — Further investigation appears to 
show conclusively that the distinction between primary and secondary seres 
is the outstanding fact of the development of existing formations. It is 
inherent in the organic nature of the formation (Chapter I), and is no more 
subjective than the reproduction by seed and propagation by offshoots in the 
case of an individual plant. The original distinction was somewhat confus- 
ing, as it placed too much weight upon the initiative process. In the case of 
erosion it was particularly difficult to determine offhand whether the new 
area was primary or secondary. The concept has now been definitized by 
basing it wholly upon development, though this basis necessarily includes 
reaction and the general influence of the denuding ageut. From the develop- 
mental viewpoint, primary and secondary seres are wholly distinct. There 
is little or no possibility of confusing one with the other. At the same time 



172 CLASSIFICATION OF SERES 

it must be recognized that a secondary sere may occasionally resemble a 
primary one very closely upon casual inspection. In rare cases, they can be 
distinguished only by the fact that the prisere has the pioneer stage, while the 
subsere begins with a late initial or subpioneer stage. Such instances are 
very rare, however, and in the vast majority of cases a subsere begins with a 
medial or subfinal stage. This occasional approximation of prisere and sub- 
sere is not an argument against the validity of the concept. In the individual 
plant an exact parallel is found in the ease of species which replace the repro- 
ductive seeds wholly or in part by propagative bulbils, the development of 
the individual being all but identical in the two cases. 

Cowles (1911:167) states that the classification into primary and secondary 
successions "seems not to be of fundamental value, since it separates such 
closely related phenomena as those of erosion and deposit, and places together 
such unlike things as human agencies and the subsidence of land." This 
objection brings out clearly the difference between the physiographic and the 
developmental views of vegetation. The former apparently makes physio- 
graphic distinctions paramount, while the latter regards development as the 
sole arbiter of the importance or value of any concept or principle. It has 
repeatedly been shown (Chapter II) that, while erosion and deposit are 
closely related physiographic processes, they are not closely related succes- 
sional phenomena. Successionally they are indeed usually antagonistic, giving 
rise to fundamentally different bare areas. On the other hand, they may occa- 
sionally be equivalent as initial causes, producing xerophytic sand areas at 
one extreme or hydrophytic swamp areas at the other. In the life-history of 
a river the erosion of upland is obviously related to deposition in lowland, 
since the material for the one comes from the other. It is clear that no such 
relation exists between the two areas in so far as succession is concerned. 
Erosion on the upland yields regularly a xerarch sere, deposition on the low- 
land a hydrarch sere. The two seres may show a developmental relation by 
terminating in the same climax, or they may belong to wholly different forma- 
tions. In either case, it is evident that the student of development is con- 
cerned with erosion and deposit only because, like a host of other agents, they 
produce initial bare areas for invasion. 

Furrer (1914:30) has criticized the distinction into primary and secondary 
succession as a "far-reaching division, based predominantly upon deductive 
reasoning, and supported by insufficient analysis derived from practical 
experience. ' ' He further regards it as questionable whether the field ecologist 
can ever fall in line with this classification. This objection seems immaterial 
in view of what has been said in the preceding paragraph. Moreover, Furrer 's 
experience in successional investigation is so very slight that little weight can 
be given his opinion of a developmental relation which has had more rigorous 
and extensive field tests than any other developmental concept except suc- 
cession itself. 

Roberts (1914:432) concludes that: 

"The terms initial and repetitive seem to be better than primary and 
secondary in conveying the idea of often-repeated successions such as are 
found in a frequently deforested area. (443) 

"It is doubtful if there is any climax representing that of the so-called 
primary succession, which might well be called the initial succession. The 



PRIMARY AND SECONDARY SUCCESSION 173 

region represents a third or fourth attempt to develop a climax forest, as do 
most of the New England forest areas. These successions have been called 
secondary successions, but might better be called repetitive associations, 
because the deforestation causes the area to revert to an aspect which is a 
combination of a former succession with the successions which ordinarily 
follow it. The term 'secondary' does not carry with it the idea of more than 
one attempt at repetition, while repetitive carries with it no limit in the 
number of attempts. " (435) 

These suggestions afford a striking illustration of the danger of generalizing 
upon the basis of a first study and that made upon a very limited area. The 
superficial fact of repetition is taken as more important than the process of 
development itself. It is not even recognized that "initial" or primary succes- 
sions are repeated again and again in the same climax, as well as in the same 
spot. Moreover, the figure on page 442 indicates that there is no essential 
difference between the stages of burn "repetitive" and "initial" successions, 
a conclusion wholly impossible under the terms of an exact quantitative 
study. 

Warming (1896:350) had already distinguished between changes in vege- 
tation due to (1) the production of new soil and (2) changes in old soil, or in 
the vegetation covering it, particularly those caused by man. While this is 
not the full or exact distinction between primary and secondary succession, 
it does include much of it. The same idea is more clearly brought out in his 
earlier distinction (1892) between primary and secondary formations, in 
which the latter comprise those due to the influence of man. Tansley (1911:8) 
and his colleagues have used this concept of primary and secondary processes 
in connection with the study of succession in British vegetation. It has been 
adopted in America by Shantz (1906:187), Jennings (1908:291; 1909:306), 
Schneider (1911:290), Dachnowski (1912:223, 257), Gates (1912, 1915), 
Cooper (1913:11), Negri (1914:14), Pool (1914:304-306), Bergman and 
Stallard (1916) and others. 

Cowles's system. — Cowles (1911:168) has classified successions as (1) re- 
gional, (2) topographic, and (3) biotic. He states that: 

"In succession, we may distinguish the influence of physiographic and of 
biotic agencies. The physiographic agencies have two aspects, namely, 
regional (chiefly climatic) and topographic. (168) In regional successions it 
would seem that secular changes in climate, that is, changes which are too 
slow to be attested in a human lifetime, and which perhaps are too slow to 
be attested in a dozen or a hundred lifetimes, are the dominating factors. 
Regional successions are so slow in their development that they can be studied 
almost alone by the use of fossils. It is to be pointed out that great earth- 
movements, either of elevation or subsidence, that is, the far-reaching and 
long-enduring epeirogenic movements, as contrasted with the oscillations of 
coast-lines, must be considered in accounting for regional successions ; the 
elevation of the Permian and the base-leveling of the Cretaceous must have 
played a stupendous part in instituting vegetative change. (170) 

"In striking contrast to secular successions, which move so slowly that we 
are in doubt even as to their present trend, are those successions which are 
associated with the topographic changes which result from the activities of 
such agents as running water, wind, ice, gravity, and vulcanism. In general, 
these agencies occasion erosion and deposition, which necessarily must have a 
profound influence upon vegetation. As might be expected, the influence of 



174 CLASSIFICATION OF SERES 

erosion generally is destructive to vegetation, or at least retrogressive, while 
the influence of deposition is constructive or progressive. (170) 

"Of less interest, perhaps, to the physiographer than are the vegetative 
changes hitherto considered, but of far greater import to the plant geographer, 
are the vegetative changes that are due to plant and animal agencies. These 
are found to have an influence that is more diversified than is the case with 
physiographic agencies ; furthermore, their influence can be more exactly 
studied, since they are somewhat readily amenable to experimental control, 
but particularly because they operate with sufficient rapidity to be investi- 
gated with some exactness within the range of an ordinary lifetime. If, in 
their operation, regional agencies are matters of eons, and topographic agen- 
cies matters of centuries, biotic agencies may be expressed in terms of 
decades. (171) 

"At first thought, it seems somewhat striking that far-reaching vegetative 
changes take place without any obvious climatic change and without any 
marked activity on the part of ordinary erosive factors. Indeed, it is prob- 
ably true that the character of the present vegetative covering is due far more 
to the influence of biotic factors than to the more obvious factors previously 
considered. So rapid is the action of biotic factors that not only the climate, 
but even the topography may be regarded as static over large areas for a con- 
siderable length of time. It has been said that many of our Pleistocene 
deposits exhibit almost the identical form which characterized them at the 
time of their deposition, in other words, the influence of thousands of years of 
weathering has been insufficient to cause them to lose their original appear- 
ance. These thousands of years would have sufficed for dozens and perhaps 
for hundreds of biotic vegetative cycles. Many a sand dune on the shores of 
Lake Michigan is clothed with the culminating mesophytic forests of the 
eastern United States, and yet the sand dunes are products of the present 
epoch ; furthermore, sand is regarded generally as a poor type of soil in which 
to observe rapid succession. If a clay upland were denuded of its forest and 
its humus, it is believed that only a few centuries would suffice for the meso- 
phytic forest to return. (172) 

"Although they grade into one another as do all phenomena of nature, we 
may recognize climatic agencies, which institute vegetative cycles whose 
duration is so long that the stages in succession are revealed only by a study 
of the record of the rocks. Within one climatic cycle there may be many 
cycles of erosion, each with its vegetative cycle. The trend of such a cycle can 
be seen by a study of erosive processes as they are taking place to-day, but 
the duration of the cycle is so long that its stages can be understood only by 
a comparison of one district with another; by visiting the parts of a river 
from its source to its mouth, we can imagine what its history at a given point 
has been or is to be. Within a cycle of erosion there may be many vegetative 
cycles, and among these there are some whose duration is so short that exact 
study year by year at a given point makes it possible to determine not only 
the trend of succession, but the exact way in which it comes about. It is 
clear therefore that vegetative cycles are not of equal value. Each climatic 
cycle has its vegetative cycle; each erosive cycle within the climatic cycle in 
turn has its vegetative cycle ; and biotic factors institute other cycles, quite 
independently of climatic or topographic changes." (181) 

In the last two statements Cowles has made evident one of the chief 
objections to a primary classification of successions as regional, topographic, 
and biotic. This is that these successions actually represent three totally 
different degrees of development or developmental sequences. His biotic 



PRIMARY AND SECONDARY SUCCESSION 175 

succession is a developmental unit, a unit succession or sere ; the topographic 
succession is a series of biotic successions, i. e., a cosere ; and the climatic cycle 
or succession is a series of coseres, i. e., a clisere or an eosere. This reveals 
the basic objection to a classification grounded upon causes. As is obvious, 
it not only obscures the developmental subordination of the three kinds of 
succession, but it also ignores the fact that so-called biotic successions may 
be caused by topography, climate, or artificial agents, man, and animals. 
These may also be agents in topographic succession as well, though less 
frequently. As has been often pointed out in the discussion of initial causes, 
the same sere or cosere may result from a number of different causes. More- 
over, climatic and topographic factors are inextricably mingled in the causa- 
tion of eosere and clisere. This is inevitable from the coincidence of deforma- 
tional, sun-spot and volcanic cycles (cf. "Plant Succession," fig. 26 and plate 
57). Furthermore, in all periods with peat or coal seres and coseres, such as 
the Pleistocene, Cretaceous, Pennsylvanian, etc., the same development may 
result from flooding due to increased rainfall or to a local sinking of the 
region. 

Another source of confusion lies in the fact that biotic succession is stated 
to be due to plants and animals. The role of plants is that of reaction upon 
the habitat, as a consequence of which one stage succeeds another. Such a 
reaction is typical of all succession, and the latter would be impossible with- 
out it. Man and animals, on the contrary, are initial causes, as is topography, 
and have little to do with reaction. Hence, as already shown (Chapter III), 
it is imperative for the understanding of vegetational development to distin- 
guish initial causes, topographic, climatic, and biotic, from ecesic or continu- 
ative causes, of which reaction is the most striking. Moreover, a plant may 
itself be an initial cause, in such instances as the one mentioned, where Cus- 
cuta produced a bare area again by completely destroying the pioneers of a 
dune sere. This confusing double use of the term biotic is well illustrated by 
the statements of Paulsen (1912:104) and Matthews (1914:143). Speaking 
of the sand desert, Paulsen says that the development from stable to unstable 
desert through the agency of man must be considered a biotic succession. 
Matthews, in describing the water sere in Scotland, states that there seems to 
be sufficient evidence for regarding the main determining factors as entirely 
biotic. In the former, the cause of the bare area is biotic, in the latter, topo- 
graphic ; in both the ensuing course of development is due to the reaction of 
plants, and is necessarily biotic. 

Crampton (1911:20; 1912:4) has adopted Cowles's classification, as have 
also Crampton and MacGregor (1913:180), but his application of the terms 
appears to be more or less divergent. The regional successions of Crampton 
seem to include the small and recent swings of climate, such as are found in 
the coseres of peat-bogs (1911:22), rather than the great eoseres of geological 
history. His topographic successions seem to be the existing ones due to local 
topographic initial causes (1911:29) and not those of Cowles, which are 
related to the vast regional changes comprised in an erosion cycle. Crampton 
appears to ignore biotic successions altogether, especially the vast number of 
secondary successions, regarding the local topographic succession as well-nigh 
universal, while Cowles ascribed much the greater importance at present to 
his biotic successions (1911:172). Crampton 's treatment is still further com- 



176 CLASSIFICATION OF SERES 

plicated by the distinction between stable or paleogeic and migratory or 
neogeic formations, which seem to correspond roughly to climax and serai 
communities respectively. It also serves to lend much emphasis to the fact 
that in the study of the development of vegetation, development is obviously 
paramount and physiography quite secondary. 

Watson's (1912:213) use of the term "biotic succession" also illustrates 
the inevitable confusion to which it leads : 

"After a fire in the Douglas spruce, the quaking aspen always takes posses- 
sion, but it has also its natural place as a transition between the oak chaparral 
and the Douglas spruce in the biotic succession. The biotic succession in the 
Sandia Mountains is as follows: The bare rock first incrusted with crustose 
lichens, then foliose lichens, mosses, herbs, oaks, followed in some cases 
directly by Douglas spruce, and in others by aspen and then the spruce ; and 
then as physiographic succession comes in, the poplars, pines, and box-elders 
in the canon and pine, pinon, and cedar on the slopes, and the ultimate forma- 
tion of the mesa is reached. ' ' 

The aspen is a characteristic stage of the secondary succession due to man 
as a biotic cause, while it progresses to the Douglas-spruce stage in conse- 
quence of the reactions of plants as biotic agents. The last is also true of the 
primary succession initiated on rock by crustose lichens, but as to cause, this 
succession is essentially topographic. 

Siegrist (1913:145) has also distinguished topographic and biotic succes- 
sions, but his topographic succession is the biotic succession of Cowles. This 
is shown by the definition of a topographic succession as one in which a topo- 
graphic change is necessary for the initiation of a new formation. The 
examples given on pages 158 and 159 further prove that he is concerned with 
local unit succession or seres, and not at all with the topographic successions 
of Cowles, which are matters of centuries and belong to far-reaching erosive 
cycles. Biotic succession is defined as one in which no topographic change is 
necessary, though it does not exclude the simultaneous occurrence of such 
changes, which, however, have no influence upon the biotic succession. The 
author's use of the term is in itself incorrect as well as misleading, as he 
employs it for parts of a unit succession or sere (I. c, 145, 158), e. g., Hippo- 

phaetum >Pinetum, Hippophaetum >Transition association, Pine- 

tum >Transition association. As already indicated, his topographic asso- 
ciations are necessarily biotic in reaction, and would be called biotic succes- 
sions by Cowles. The distinction made on page 159 is far from evident, but 
it seems to be based upon whether colonization takes place in the water or 
upon a new area of sand or gravel. Prom the standpoint of development, 
a pond or stream is just as much a bare area due to a topographic initial 
cause as is a sand-bar or a gravel-bank, and the succession on each proceeds 
as a consequence of the biotic reactions of the plants. It is also difficult to 
understand how local topographic- changes can occur without initiating or 
affecting succession. 

Dachnowski (1912:259) has distinguished two kinds of successions, as 
follows : 

"Two great, relatively wave-like and integrating phases of vegetation suc- 
cessions define themselves rather clearly: (1) the climatic successions, asso- 
ciated with the succession of geological periods and of which the migration of 



BASES OF CLASSIFICATION 177 

plants accompanying and following the retreat of the glaciers is an example ; 
(2) the edaphic successions, in which the replacement of one type of vegeta- 
tion by another has resulted from changes in topography and a bio-chem- 
ically diminished water-supply." 

The climatic successions are the regional successions of Cowles, and the 
edaphic ones correspond partly to his topographic and partly to his biotic 
successions, thus emphasizing the impossibility of distinguishing between the 
two on the grounds proposed. Dachnowski's climatic successions would 
include the geosere, eoseres, and cliseres and coseres in part, though deforma- 
tion and gradation play a profound role in them. His edaphic successions 
would correspond partly to the cosere and sere. According to the definition 
given, seres due to biotic initial causes would find no place in either group. 
In short, the distinction proposed, like all of those based upon initial causes, 
runs counter to the process of development, and hence is largely artificial. 

Braun and Furrer (1913:19) use the term phylogenetic successions for the 
regional successions of Cowles, though this term should obviously include his 
topographic successions as well. Contrasted with this is the ontogeny of 
actual communities, which establish themselves under the eyes of the observer. 
These apparently correspond exactly to the biotic successions of Cowles, 
though the authors ignore this fact, and distinguish artificial successions, 
equivalent to Cowles 's retrogressive biotic successions. 

Possible bases of classification. — From the preceding discussion it becomes 
clear that development, cause, initial area, and climax must be weighed as 
possible bases for the classification of successions. Reaction is not available, 
since one sere is often the result of several reactions, and since widely differ- 
ent seres may have the same sequence of reaction. Since the reaction upon 
water-content is nearly universal in succession, classification may be based 
upon the direction of movement, such as mesotropic, xerotropic, etc., but our 
present knowledge hardly suffices for this. 

In a natural, i. e., a developmental system of classification, it is clear that 
development must constitute the chief basis. This is true of the actual seres 
of to-day, which culminate in the present climax formation. It is true of the 
cliseres, which result from the shifting of existing climaxes, and of the coseres 
formed by successive seres. It is even more marked in the eoseres, which are 
major developmental series within the climatic climax of the geological eras. 
In short, seres are related to each other by their development into the same 
climax and by their sequence in the cosere. Climaxes, the static units of 
to-day, are related to each other in the developmental sequence of the clisere, 
which is produced by a change of climate, such as glaciation. These climaxes 
of the existing flora are phylogenetically the descendants of the climaxes of a 
preceding flora, which characterized an eosere. All eoseres have a similar 
phylogenetic relationship, and taken together constitute the geosere, the whole 
course of the development of vegetation from its beginning down to the 
present. 

Developmental basis of classification. — While the unique importance of 
development for successional analysis and classification has repeatedly been 
emphasized, it is felt that over-emphasis is impossible. Though it is easy to 
carry analogy too far, there seems to be no question that the history of ecology 
must repeat that of botany itself to a large degree. In morphology and tax- 
is 



178 CLASSIFICATION OF SERES 

onomy, development alone is regarded as capable of furnishing basic criteria, 
and the great advances in these fields are regarded as necessarily consequent 
upon an increased knowledge of development. What is true of the individual 
and the species seems equally true of the community. Studies of physiognomy, 
floristic, and habitat all have their importance, but their chief value lies 
in their correlation into the basic process by which communities arise and 
grow, namely, development. It is evident that our knowledge of develop- 
ment will advance more slowly than it will in the three fields just mentioned, 
but it is also clear that the final importance of any advance will depend upon 
its developmental significance. 

The natural classification of seres rests upon the fact that each sere leads to 
a climax or formation. Hence, the fundamental grouping of seres is deter- 
mined by their relationship to a particular formation. As a consequence, all 
the seres of one formation constitute a natural group, strictly homologous 
with all the seres of another formation. Thus, all the existing seres of the 
world fall into as many coordinate groups as there are climatic climaxes in 
vegetation. In short, the primary division in a natural classification of seres 
is that into climaxes or formations. As previously indicated, the latter fall 
into the major developmental groups of clisere and cosere. While formations 
may also be arranged in formation groups, classes, or types, for convenience 
of reference, such groupings seem unfortunate in that they tend to postpone 
a natural classification (plate 21, a, b). 

The grouping of seres within each formation should also be based upon 
development. The reasons for the distinction of primary and secondary seres 
have been discussed at length (pp. 60, 169), and it is only necessary to empha- 
size the fact that these represent the basic developmental differences within 
the formation. The actual recognition of priseres and subseres is a simple 
matter, except occasionally in the final stages which are converging into the 
climax. The distinction between primary and secondary bare areas is readily 
made as a result of experience in successional investigation, though it should 
always be checked by instrumental study. The only possible difficulty with 
the division into prisere and subsere arises when the secondary disturbance 
is so profound as to cause the resulting area to approach the condition of 
a primary one. The difficulty here, however, is not one of distinguishing 
prisere and subsere, since the distinction between them is clear-cut. The 
prisere repeats the whole course of normal development, the subsere retraces 
only a part of it. The subsere regularly comprises the later half or less of the 
succession. While it may exceptionally begin at an earlier point, its initial 
stage is always subsequent to the pioneer associes of the prisere. In short, a 
subsere can never begin on an initial bare area of rock, water, or sand unless 
the effects of plant reaction are already manifest in it. 

Initial areas and causes. — It has already been shown that the significance 
of initial areas for succession lies in the conditions as to water-content, and 
not in their causes. Since the initial water-content is determined in some 
degree by the initial cause, the latter may be used as the basis for subdivisions. 
In this connection, however, it is necessary that the causes themselves be con- 
sidered and grouped from the standpoint of their effect upon water-content, 
and not from that of their nature. Such a classification would regularly sepa- 
rate erosion and deposit by water, since the one produces relatively dry and 



CLEMENTS 



PLATE 21 




A. Deciduous forest climax of Aoer-Fagws, Three Oaks, Michigan. 

B. Subalpinc forest climax of Picca-Abies, Mount Blanca, Colorado 



THE CLIMAX AS A BASIS 179 

the other relatively wet initial areas. It would bring them together when they 
produce essentially the same area, as is frequently true of wind erosion and 
deposit, and not altogether rare in the case of water. While the value of the 
initial area for purposes of classification rests upon its water-content, it must 
not be forgotten that the nature of the latter may be more significant than its 
amount. In other words, an alkaline or acid holard may determine the nature 
of the sere, more or less irrespective of the amount of water present. 

The initial causes of bare areas are largely or predominantly physiographic. 
Their role in succession is not due to their nature as physiographic processes, 
but to their effect upon water-content. As indicated above, this effect is due 
in some degree, and often a controlling one, to the nature of the agent. This 
relation is not so definite, however, that the process can be substituted for 
water-content as a basis of classification. Thus, while it is clear that a com- 
plete study of succession must include the causes which initiate seres, it 
assigns to physiography a subordinate role in classification as in development. 

Relative importance of bases. — The basic division of the developmental 
classification of seres here proposed is the climax or formation. Every climax 
is subdivided into priseres and subseres, each with a larger or smaller number 
of adseres. Priseres and subseres are further grouped with reference to the 
initial water-content of the bare area, in the manner indicated by Cooper's 
distinction into hydrach and xerarch seres. Finally, these may be further 
divided into groups based upon the causes which produce a particular bare 
area. Such a classification is developmental throughout, since even the minor 
divisions based upon initial causes have this value, if the causes are grouped 
in accordance with their action rather than their nature. 

The climax as a basis. — The nature of the climax as the final condition of 
the vegetation of a climatic region through a climatic period makes unavoid- 
able its use as the primary basis for the classification of existing seres. The 
use of the climax necessarily depends upon its recognition, and this is a mat- 
ter of some difficulty in the present state of our knowledge. Neither clima- 
tology nor ecology has reached a point at which climatic climaxes can be 
delimited accurately. In fact, climatology is obviously of secondary impor- 
tance in this connection. While it is perhaps easier to study climate than 
vegetation, it is the latter alone which makes possible the recognition of a 
particular climate so far as plants are concerned. In other words, a climax 
must be determined by its developmental and structural character, as is true 
of any biological unit. This is true in spite of the fact that climate is the 
cause of a climax, or at least the force in control of it. 

In the United States and Europe the developmental study of vegetation has 
gone far enough to disclose a large number of seres. This has had the effect 
of delimiting in a general way the majority of climaxes on the two continents, 
first by determining the successional termini of the various regions, and 
secondly, by making it possible to distinguish between serai stages, associes 
and consocies on the one hand, and ultimate communities, associations and 
consociations on the other. The result has been to confirm the general 
floristic evidence as to the existence and extent of climaxes, though the limits 
and relations of these are still to be determined with precision. 

Recognition of climax areas.— All the attempts to divide the surface of 
the earth into vegetation zones or climatic regions have some bearing upon 



180 CLASSIFICATION OF SERES 

the problem of climax areas. The various divisions of North American vege- 
tation by Gray (1878), Engler (1879), Sargent (1880), Drude (1887), 
Merriam (1898), Clements (1904), Harshberger (1911), and others, have 
either been based more or less completely upon the basic climax units, here 
regarded as formations, or at least represent them in some degree. Thus, 
while there is the usual divergence of view as to the basis, relationship, and 
terminology of the various subdivisions, there is necessary agreeement as to 
the actual existence of a more or less definite number of distinct vegetation 
areas. Few attempts have been made to investigate these as climaxes and to 
determine their limits, relations, and development. Cowles (1899, 1901) and 
Whitford (1901) have considered the general relation of development to 
climax in the forested region of Illinois and Michigan. Adams (1902:128) 
has sought to lay down general rules for the study of life centers, in connec- 
tion with a study of the southeastern United States as a center of dispersal 
and origin : 

"First. In general the fauna and flora of northern United States east of 
the Great Plains are geographically related to those of the Southeast and this 
geographical relationship points to an origin in the direction of the Southeast 
except in the case of the distinctly boreal forms. 

"Second. The abundance and diversity of life in the Southeast indicate 
that it has been, and now is, a center of dispersal. 

' ' Third. The relicts indicate that the Southeast has been a center of preser- 
vation of ancient types, and the endemism shows that it has been a center of 
origin of types. 

' ' Fourth. There are two distinct southern centers of dispersal in temperate 
United States; one in the moist Southeast, and the other in the arid South- 
west. 

"Fifth. Ten- criteria, aside from fossil evidence, are recognized for deter- 
mining the center of origin or the locality of dispersal : 

"1. Location of the greatest differentiation of a type. 

"2. Location of dominance or great abundance of individuals. 

"3. Location of synthetic or closely related forms. (Allen.) 

"4. Location of maximum size of individuals. (Ridgway, Allen.) 

"5. Location of greatest productiveness and its stability, in crops. 
(Hyde.) 

"6. Continuity and convergence of lines of dispersal. 

"7. Location of least dependence upon a restricted habitat. 

"8. Continuity and directness of individual variations or modifica- 
tions radiating from the center of origin along the highways 
of dispersal. 

"9. Direction indicated by biogeographical affinities. 
"10. Direction indicated by annual migration in birds. (Palmen.) 
' ' Sixth. There are three primary outlets of dispersal from the Southeast : 

"1. The Mississippi Valley and its tributaries. 

"2. The Coastal Plain. 

"3. The Appalachian Mountains and adjacent plateaus. 

"The first two have also functioned for tropical types, and the third for 
boreal forms. Dispersal is both forward and backward along these highways. 

"Seventh. The individual variations of animals and plants, such as size, 
productiveness, continuity of variation, color variation, and change of habit 
and habitats, should be studied along their lines of dispersal and divergence 



THE CLIMAX AS A BASIS 181 

from their center of origin. Life areas should be studied as centers of dis- 
persal and origin, and hence dynamically and genetically." 

In studying the forest vegetation of eastern America by plotting the ranges 
of dominant trees, Transeau (1905:886) confirms the results of earlier ob- 
servers as to the existence of four distinct forest centers, namely : 

"(1) The Northeastern conifer forest centering in the St. Lawrence basin, 
(2) the deciduous forest, centering in the lower Ohio basin and Piedmont 
plateau; (3) the Southeastern conifer forest, centering in the south Atlantic 
and Gulf Coastal plain; and (4) the insular tropical forest of the southern 
part of the Florida peninsula, centering in the West Indies. The term center, 
as here used, implies the idea of distribution about a region where the plants 
attain their best development. Such vegetation divisions are not fixed, but 
move and increase or decrease in extent depending upon continental evolution 
and climatic change. 

"It has been found that if the ratios, produced by dividing the amount of 
rainfall by the depth of evaporation for the same station, be plotted on a map, 
they exhibit climatic factors which correspond in general with the centers of 
plant distribution. Further, the distribution of grassland, prairie, open forest, 
and dense forest regions is clearly indicated. This is explained by the fact 
that such ratios involve four climatic factors, which are of the greatest impor- 
tance to plant life, viz, temperature, relative humidity, wind velocity, and 
rainfall. ' ' 

Recently, Livingston (1913:257) has integrated the temperature and 
moisture relations of the climatic areas, and has developed a general method 
of determining the climatic control of climax formations. We are still far 
from the final method for delimiting climaxes and their climates. It seems 
clear, however, that it must be based primarily upon the range of consocia- 
tions, and upon the measurement of the growth and reproduction of their 
dominants in relation to the water and temperature conditions of both the 
growing and resting periods. 

Climaxes of North American vegetation. — Clements (1902:15; 1904:160) 
has made an analysis of North American vegetation upon the basis of temper- 
ature and water zonation, in an endeavor to determine the great vegetation 
centers. The major continental zones were thought to be due to temperature 
and water, and their interruption to the decreasing rainfall and increasing 
evaporation toward the interior, as well as to the disturbing effect of moun- 
tain ranges. The 17 provinces were supposed to indicate as many vegetation 
centers, but they were determined floristically, by the superposition of the 
ranges of dominants, and not developmentally. Hence, while most of them 
correspond to climax formations, some obviously do not. With the recog- 
nition of the formation as the major unit, of vegetation, the question of zones, 
regions, provinces, etc., becomes of minor importance. These are geograph- 
ical distinctions based upon floristic, while the developmental method 
demands vegetation distinctions based upon climaxes and the course of suc- 
cession. 

The first division of the vegetation of North America into climax forma- 
tions was made in "Plant Succession" (page 180) and was necessarily based 
upon field work done before 1915. It was recognized at that time that the 
existing knowledge permitted only a tentative outline, and in consequence 
especial attention was devoted to the recognition and (continued on page 1S4) 



182 CLASSIFICATION OF SERES 

CLIMAXES OF NORTH AMERICA 

GRASSLAND CLIMAXES: 

Grassland: Stipa-Bouteloua Formation 

1. True Prairie: Stipa-Sporobolus Association 

la. Subclimax Prairie: Andropogon Associes 

2. Coastal Prairie: Stipa- Andropogon Association 

3. Mixed Prairie : Stipa-Bouteloua Association 

3a. Short-grass Plains: Bulbilis-Bouteloua Associes 

4. Desert Plains : Aristida-Bouteloua Association 

5. Pacific Prairie : Stipa-Poa Association 

6. Palouse Prairie: Agropyrum-Festuca Association 
Sedgeland (Tundra) : Carex-Poa Formation 

1. Arctic Tundra: Carex-Cladonia Association 

2. Petran Tundra: Carex-Poa Association 

3. Sierran Tundra: Carex-Agrostis Association 

SCRUB CLIMAXES: 

Sagebrush: Atriplex-Artemisia Formation 

1. Basin Sagebrush: Atriplex-Artemisia Association 

2. Coastal Sagebrush : Salvia- Artemisia Association 
Desert Scrub: Larrea-Franseria Formation 

1. Desert Scrub : Larrea-Franseria Association 
la. Bronze Scrub: Larrea-Flourensia Associes 
lb. Mesquite: Acacia-Prosopis Associes 
lc. Sotol: Agave-Dasylirion Associes 
Id. Thorn Scrub: Cereus-Fouquiera Associes 
Chaparral: Quercus-Ceanothus Formation 

1. Petran Chaparral: Cereocarpus-Quercus Association 

la. Oak-sumac Subclimax: Rhus-Quercus Associes 

2. Coastal Chaparral: Adenostoma-Ceanothus Association 

FOREST CLIMAXES: 

Woodland: Pinus-Juniperus Formation 

1. Pinon-juniper Woodland: Pinus-Juniperus Association 

2. Oak-juniper Woodland: Quercus-Juniperus Association 

3. Pine-oak Woodland: Pinus-Quercus Association 
Montane Forest: Pinus-Pseudotsuga Formation 

1. Petran Montane Forest: Pinus-Pseudotsuga Association 

2. Sierran Montane Forest : Pinus-Abies Association 
Coast Forest: Thuja-Tsuga Formation 

1. Cedar-hemlock Forest: Thuja-Tsuga Association 

la. Douglas-fir Subclimax: Pseudotsuga Consocies 

2. Larch-pine Forest : Larix-Pinus Association 
Subalpine Forest: Picea-Abies Formation 

1. Petran Subalpine Forest : Picea-Abies Association 

la. Lodgepole Subclimax: Pinus consocies 

2. Sierran Subalpine Forest: Pinus-Tsuga Association 
Boreal Forest: Picea-Larix Formation 

1. Spruce-larch Forest: Picea-Larix Association 

la. Birch-aspen Subclimax: Betula-Populus Associes 

2. Spruce-pine Forest: Picea-Pinus Association 
Lake Forest: Pinus-Tsuga Formation 

1. Pine-hemlock Forest: Pinus-Tsuga Association 
la. Jack-pine Subclimax: Pinus Consocies 
Deciduous Forest: Quercus-Fagus Formation 

1. Maple-beech Forest: Acer-Fagus Association 

2. Oak-chestnut Forest: Quercus-Castanea Association 

3. Oak-hickory Forest: Quercus-Hicoria Association 

la. Pine Subclimax: Pinus Associes 

Isthmian Forest 
Insular Forest 



THE CLIMAX AS A BASIS 183 

SUBCLIMAXES: 

The major advances made in the past fourteen years have consisted largely in 
the more exact delimitation of subclimaxes. At the time of the first treatment, 
it was still uncertain how much of the prairie proper was to be regarded as sub- 
climax, though by 1920 it had become evident that the true prairie comprised the 
larger part of it. In the later arrangement, the short-grass plains were still 
treated as climax, but the study of relict and protected areas, and in particular of 
experimental exclosures, has demonstrated that they constitute a subclimax caused 
by grazing. The detailed investigation of climaxes in Texas has revealed a new 
association, the coastal prairie, related to both the subclimax and true prairies 
(Tharp, 1926; Clements and Tharp, 1926). Similarly, a more intimate acquaintance 
with the bunch-grass prairie of the Pacific Coast has made it desirable to divide 
this into the Pacific and Palouse prairies, owing to the fact that the chief dominants 
are different. Finally, the field experiments carried out in connection with "Experi- 
mental Vegetation" and "Plant Competition" have furnished objective values to 
these new viewpoints. 

The understanding of the desert scrub has been greatly promoted by resident 
studies in Arizona and California during the last decade. The outcome has been 
the recognition of a single climax association characteristic of the desert climate 
of the lower Colorado Basin. With it are associated four subclimaxes, the first 
two being due primarily to overgrazing and fire and the last two related to rocky 
foothills and escarpments. 

It is as significant as it is interesting to note that the chaparral and forest 
climaxes all exhibit subclimaxes, with the exception of woodland, a large part of 
which is itself subclimax today. The lodgepole subclimax is placed in the subalpine 
forest, but it also extends well into the zone of the montane forest. 

Names of climaxes. — A consistent endeavor has been made to render the nomen- 
clature of formations and associations as simple, uniform and definite as possible. 
In the tentative arrangement first proposed it seemed desirable to prepare the way 
for an international nomenclature by means of technical terms for the two major 
divisions, such as hylion for a forest formation and hylium for an association. 
These terms still have distinct values and may come into more general use, as 
developmental studies become more or less universal. However, the use of the 
words formation and association is becoming more and more the rule, and this 
may well make the less familiar hylion, dry on, poion, etc., unnecessary. 

The nomenclature exemplified in the present classification of climaxes is essen- 
tially binomial in character. Moreover, it comprises two sets of names, the one 
technical and adapted to international usage, the other vernacular and suited to 
more general purposes. The former are limited to the names of the two most 
important dominants, or even to a single one when the chief dominants belong 
to the same genus. The endeavor has been made to attain the optimum with respect 
to brevity, definiteness, uniformity, and attractiveness. 

Extent and relationship of the climaxes.— -The general position and extent of 
the climaxes of western North America have been indicated in Chapter IV of "Plant 
Indicators." They are exhibited graphically in the vegetation map of the United 
States by Shantz and Zon (1924), which is in close accord with the earlier classi- 
fications indicated above. Because of the practical needs of agriculture, grazing 
and forestry, the names of the communities follow more nearly the everyday usage 
in these fields, but the genera listed for each make it possible to refer the divisions 
of the map to the proper formations and associations. 

Much progress has been made in the study of the phytogeny of the climaxes of 
the North American continent, both with respect to their relationship to each 
other and to the similar formations of Eurasia. It is even more evident than it 
was fifteen years ago that the phylogeny of climaxes must provide the key to an 
adequate understanding and treatment of them, such as is proposed in the books 
now under way or contemplated. 



184 CLASSIFICATION OF SERES 

delimitation of formations and associations during the next decade. The first 
fruits of this were to be seen in the grouping of climaxes in ' ' Plant Indicators" 
(page 114), a classification that eight years of further research have confirmed 
in nearly all the details, A comprehensive monograph of the climaxes of North 
America is in preparation, as indicated in the preface, but it is thought that 
the list of formations and associations, as shown on page 182, will prove both 
suggestive and useful meanwhile. 

Priseres and subseres. — Within the same climax, seres are classified as pri- 
mary and secondary, i. e., as priseres and subseres. The fundamental value of 
this developmental distinction has been sufficiently dwelt upon. In the actual 
study and classification of the seres of any climax, the significance of the dis- 
tinction will be obvious in the vast majority of cases. Subseres will be found 
chiefly confined to bare areas due to superficial and usually artificial disturb- 
ance, especially as a consequence of man's activities. They are much more 
numerous than priseres, and are much more readily investigated, since the 
persistence of the preceding reactions causes succession to go forward rapidly. 
In the grassland and forest stages the associes are often normal, and thus 
throw much light upon the slower but corresponding stages of the primary 
succession. Priseres are typical of the three extreme areas, water, rock, and 
sand, in which no effective reaction has occurred. In the case of sand, this is 
perhaps true only of primary dunes, in which the extreme condition due to 
complete lack of humus and to low surface water-content is reinforced by the 
great instability. Priseres are relatively infrequent in great midland regions 
of forest and grassland, but they are sufficiently common to furnish a reliable 
comparative basis for the study of succession. In lowland and montane 
regions examples of priseres are often more numerous than those of subseres, 
and such regions are of the first importance for serai investigations (plate, 
22, A, b). 

Hydroseres and xeroseres. — It has already been suggested that the water- 
condition of the initial area furnishes a better basis for the subdivision of 
priseres and subseres than does the water-content of the climax. A complete 
classification upon the basis of water relations would require a primary divi- 
sion into hydrotropic, xerotropic, and mesotropic seres, but the latter are so 
overwhelmingly predominant in the present state of our knowledge that they 
alone demand consideration. With increasing study of desert and tropical 
succession it is probable that the direction of the water-reaction will assume 
its basic importance. At present, however, it is most convenient to regard 
seres as primarily mesotropic, and to distinguish them as hydrarch and xer- 
arch in accordance as they arise in wet or in dry areas (Cooper, 1912:198). 
For the sake of uniformity in classification, the corresponding terms hydrosere 
and xerosere are here proposed. 

In the case of subseres, extreme conditions of water-content are rare or 
they persist for a brief period only. Hence it is sufficient to recognize but 
the two subdivisions, hydrosere and xerosere. With priseres the extremes are 
marked, and the quality of the water-content often becomes controlling also. 
As a consequence, it seems desirable to distinguish hydroseres, as haloseres 
(Gr. aA.s, clAo's, salt) and oxyseres (6£vs, acid), with the corresponding terms, 
halarch and oxarch. It must be recognized, however, that haloseres and 
oxyseres are properly adseres, since they depart from the normal development 



CLEMENTS 





A. Prisere altemes showing the sera] stages from the bare diatom marsh to the 

lodgepole subclimax, Firehole Basin, Yellowstone Park. 

B. Subsere altemes duo to the removal of sods for adobe houses, showing three 
stages: (1) rushes, (2) salt-grass, (3) Anemopsis, Albuquerque, Now Mexico. 



CLASSIFICATION OF SERES 



185 



only for a portion of the sere. Moreover, while the surfaces of rock and of 
dune-sand may be almost equally dry, the differences of hardness and stability 
result in very dissimilar adseres. These may be distinguished as lithoseres 
(Gr. XlOos, rock) and psammoseres (Gr. ipa^o*, sand), or as litharch and 
psammarch. Finally, hydroseres and xeroseres may be also distinguished 
upon the basis of the agents concerned in producing bare areas. While this 
has value in connection with the origin of such areas, it is not fundamental, 
and hence is out of place in a developmental 
classification (plate 23, A. b). 

Phylogenetic system. — The arrangement 
proposed above deals with the grouping of 
seres within a particular climax. It applies 
to the relations of existing seres, as well as 
to those of each period or era and sums up 
the ontogeny of the climax formation. The 
phylogenetic relations of the latter obviously 
must be sought in the geological past. They 
serve to show the immediate origin of the 
climaxes of to-day, and to summarize the lines 
of vegetational descent in the remote past. 
The complete system of classification is shown 
in the accompanying outline. 



Geosere. 
Eosere. 
Clisere. 
Cosere. 

Sere (climax). 
Prisere. 
Hydrosere. 
Halosere. 
Oxysere. 
Xerosere. 
Lithosere. 
Psammosere. 
Subsere. 
Hydrosere. 
Xerosere. 



X. THE INVESTIGATION OF SUCCESSION. 

Primary methods. — There are three primary methods of investigating suc- 
cession: (1) by inference; (2) by sequence, (3) by experiment. Investigation 
by inference consists in piecing together the course of development from the 
associes and consocies found in a region. From the very nature of succession, 
this method was necessarily the first one to be employed, and its use still 
predominates to the practically complete exclusion of the other two. This 
is easily understood when one recalls that it is the only method that can be 
applied in studies lasting but one or two seasons. Moreover, the interpreta- 
tion of successional evidence has reached a point where inference often yields 
fairly conclusive results, and regularly furnishes the working hypotheses to 
be tested by the methods of sequence and experiment. In a complete system 
of investigation, inference can only furnish the preliminary outline, which 
must be subjected to thoroughgoing test by means of sequence and experiment 
before the course of succession can be regarded as established. However, it 
must be recognized that the value of inference, even when used alone, must 
steadily increase in just the degree that it is confirmed by the other two 
methods. Successional studies have been slow in making their way in ecology, 
in spite of their fundamental value, because of the labor and time demanded 
even by the method of inference. The adoption of the more conclusive and 
exacting methods of sequence and experiment will be slower still, but there 
would seem to be no serious doubt of their final and complete acceptance. 

The method of sequence consists in tracing the actual development of one 
or more communities in a definite spot from year to year. In short, it is the 
direct study of succession itself as a process. It is clear that sequence must 
furnish the basic method of study, and that the value of inference and experi- 
ment depends upon the degree to which they reveal the sequence itself. If 
the whole course of development from bare area to climax required but a few 
years, or a decade or two at most, the method of sequence would give us a 
complete account of succession. But even the shortest of secondary seres 
require a decade or longer, and most of them demand more than the working 
period of a life-time. Primary seres rarely if ever complete their development 
within a century, and the large majority of them last through several cen- 
turies, or even millenia. As a result, the method of sequence can not be ap- 
plied directly by a single investigator to the whole course of development from 
the pioneer colonies in water or on rock to the final grassland or forest climax. 
Three possible solutions present themselves, however. He may carry his 
studies of a particular community as far as possible, and then turn his records 
of the development over to a younger investigator, who will carry the record 
through another life-time. Such a method requires concerted action such as 
is unknown at present, but there can be little question that continuous investi- 
gations of this nature will soon be organized by great botanical institutions. 
In fact, an approach to it has already been made by the Desert Laboratory of 
the Carnegie Institution of Washington and by some of the experiment sta- 
tions of the United States Forest Service. So far, however, research is chiefly 
a function of the individual investigator, and he will seek one or both of the 
186 



CLEMENTS 





A. Hydrosere of Batrachvum, Potamiogeton, Nymphaea, Carex, etc., Lily Lake, 

Estes Park, Colorado. 
B. Xerosove of lichens, mosses, annuals ami grasses on lava ridges, Death Valley, 

California. 



THE QUADRAT METHOD 187 

other solutions. The most obvious one is to make a simultaneous study of the 
development of what appear to be different stages of the same sere. In this 
way the whole course of succession may actually be traced in a few years by 
the same individual. The one difficulty lies in properly articulating the differ- 
ent portions thus studied, and here he must call inference to his aid, or, what 
is better, make a special synchronous investigation of the actual development. 
between every pair of stages. As a matter of fact, intensive investigation 
of this sort makes it evident that he must avail himself of both sequence and 
experiment wherever possible. The complete method, then, begins with 
inference, but rests primarily upon sequence, reinforced to the highest degree 
by experiment. 

The method of experiment is a highly desirable, if not an indispensable 
adjunct to the method of sequence. Its great value lies in the fact that it 
makes it possible to reproduce practically any or all portions of the course of 
development, and to keep them under intensive observation. Its use is 
imperative in climax areas which show few or widely scattered serai communi- 
ties, while it greatly reduces the period necessarry to secure conclusive results 
in an area where developmental stages predominate, as in some mountain 
regions. It is especially dependent upon the quadrat method, and will be 
further discussed in that connection. 

Special methods. — The special methods of successional investigation may 
be grouped under four heads, viz, (1) quadrat method, (2) mapping, (3) in- 
strumentation, (4) recording. All of these are intensive in nature and in 
purpose, with the exception of large-scale mapping, and hence find their use 
in connection with the general methods of sequence and experiment. The 
quadrat method is the essential basis of them all, and may alone suffice for the 
study of development pure and simple. The latter can not be understood, 
however, without a thorough analysis of the habitat and the plant reactions 
upon it, and for such work instruments are indispensable. Moreover, much 
ecological work has failed of its purpose for the lack of an adequate method of 
record. Such a record becomes all the more imperative with the increase of in- 
tensive investigation, and it must soon come to be recognized that no succes- 
sional study is complete without a detailed record of observation and experi- 
ment. This record should be wholly separate from its interpretation, a result 
which can be secured only by the impersonal methods of quadrating and instru- 
mentation. Mapping is primarily a method of record, but it is also possible to 
use it in connection with quadrat and instrument for purposes of investigation. 

THE QUADRAT METHOD. 

Concept and significance. — The quadrat method is regarded as comprising 
all the exact methods of determining the composition and structure of plant 
communities, irrespective of the shape or size of the measure. While no defi- 
nite line can be drawn between methods of quadrating and mapping, the latter 
is here considered to be upon a scale which does not permit dealing with indi- 
viduals, and hence mapping must confine itself to the distribution and rela- 
tions of communities. It is clear that the two may be used conjointly in the 
same area and combined in the final record, as is shown later in the methods 
employed by the Botanical Survey of Minnesota. While this basic method of 
successional study is named from the most important measure, the quadrat. 



188 THE INVESTIGATION OF SUCCESSION 

it includes also the transect, bisect, and migration circle. Though all of these 
differ in form and to some degree in purpose, they are alike in being based 
upon the enumeration or charting of the individuals of a community within a 
circumscribed area, and in disclosing as well as registering the changes in 
population and structure which are the record of development. 

The use of squares for purposes of enumeration or of determining the 
amount of plant material produced has occurred occasionally for a century or 
more (Sinclair, 1826; Darwin, 1859; Hanstein, 1859; Blomquist, 1879; Stebler 
and Schroter, 1883-1892; etc.; cf. Schroter, 1910:117). It was organized into 
a definite system for the study of the structure and development of vegetation 
by Pound and Clements (1898 2 :19; 1900:61) and Clements (1904; 1905:161; 
1907:202; 1910:45; cf. also, Thornber, 1901:29). It has since been used by 
Sernander (1901), Jaccard (1901), Oliver and Tansley (1905), Shantz (1906, 
1911), Young (1907), Sampson (1908, 1915), Spalding (1909), Raunkiaer 
(1909), Gleason (1910), Howe (1910), Tansley (1911), Pallis (1911), Adam- 
son (1912), Cooper (1913), Priestley (1913), Kearney (1914), Pool (1914), 
Weaver (1914, 1915), Hofmann (1916), Bergman and Stallard (1916), and 
others. With the rapid increase in the number of suceessional studies, the use 
of the quadrat and its modifications may be expected to become as universal 
as it is fundamental. 

Kinds of quadrats. — Quadrats are distinguished with respect to their pur- 
pose, location, or size. From the standpoint of purpose and use, they are 
divided into list, chart, permanent, and denuded quadrats. As to their loca- 
tion in a community or kind of community, they are known as layer, soil, 
water, lichen, moss quadrats, etc. The unit quadrat is taken as 1 meter 
square. This may be divided into subquadrats of a decimeter or a centimeter 
square, or grouped into perquadrats up to 100 or even 1,000 meters square. 
The meter quadrat is the unit for herbland, herbaceous layers, and grassland, 
the 10-meter for scrub, and the 100-meter for forest. List quadrats are chiefly 
useful for taking a census of individuals, species, or life-forms, and making 
floristic comparisons. Chart quadrats are primarily to record composition 
and structure, while permanent and denuded quadrats are especially designed 
for the study of succession by the methods of sequence and experiment. 

List quadrat. — The list quadrat is of slight value for the study of succes- 
sion, since the latter demands the actual study and record of a definite area 
from year to year. It serves for the superficial values of reconnaissance, but 
is of small use for intensive investigation. Its chief interest lies in the fact 
that it was the pioneer quadrat method, and that it has given rise to two appli- 
cations, which have met with some favor. These are the methods of Jaccard 
and of Raunkiaer, both designed to permit a more exact comparison of local- 
ities, communities or regions upon the basis of floristic, and of physiognomy 
also, to a certain extent. Jaccard (1901, 1902, 1908, 1912, 1914) has made use 
of the list quadrat to establish a statistical method for floristic, with especial 
reference to the origin of the flora of a region. His method is based upon : 
(1) the coefficient of community, or the degree of similarity of composition 
between the different portions of the same region ; (2) the degree of frequence 
of each species; and (3) the generic coefficient, or the percentage relation of 
the number of genera to the number of species (cf. Drude, 1895:17; Pound 
and Clements, 1900:59, 63). The application of Jaccard 's principles to pres- 



THE QUADRAT METHOD 189 

ent-day succession is difficult if not impossible, but it will perhaps serve as a 
valuable aid in tracing the differentiation of the flora of an era into climaxes, 
and the migrations of genera and species during a clisere. To serve this pur- 
pose, however, it must be modified to take account of existing differences of 
composition and structure due to development. 

Raunkiaer (1905; 1910:171; cf. Smith, 1913:16) has established a new 
system of life-forms or growth-forms based primarily upon the nature and 
degree of bud protection during the unfavorable season of the year. The 
author justly regards the use of a single criterion as more satisfactory in that 
it permits definite comparisons, and enables one to correlate life-forms and 
climate much more accurately. The analysis of the flora of any region into 
its life-forms gives a biologic or phy to-climatic spectrum, which is compared 
with a theoretical norm called the normal spectrum. This method is also ap- 
plicable in some degree to communities in connection with Raunkiaer 's use of 
the list quadrat (1909:20), later modified into a circle (1912:45). This has 
the advantages and disadvantages of the list quadrat, but its chief drawback 
lies in its failure to take account of succession. Its values are floristic alone, 
and the intensive worker will quickly pass to the more thoroughgoing methods 
of quadrating. 

Chart quadrat. — Chart quadrats differ from permanent and denuded ones 
which are also recorded in charts, only in the fact that they are not fixed and 
visited from j r ear to year. The manner of charting is the same in all (Clem- 
ents 1905:167; 1907:206). The area desired, usually a meter or 10 meters, is 
staked out by means of quadrat tapes a centimeter wide and divided into 
centimeters, with eyelets at decimeter or meter intervals. The tapes are 
fixed by means of wire stakes, with loops at the upper end by which they 
are readily moved. The end tapes are placed to read from left to right, 
and the side tapes from top to bottom. After the quadrat is squared, the 
bottom tape is placed parallel to the top one, thus inclosing a strip a decimeter 
or meter wide for charting. This is charted decimeter by decimeter from left 
to right, and the upper tape is then moved to mark out the second strip for 
charting. The two cross-tapes are alternated in this fashion until the entire 
quadrat is plotted. 

Special quadrat sheets are used for plotting (figs. 4, 5), which is always 
begun at the upper left-hand corner of the chart, the small squares aiding in 
determining the proper location of every plant. Each individual is indicated 
whenever possible, but mats, turfs, mosses, and thallus plants are outlined in 
mass as a rule. This is also done with large rosettes, bunches, and mats, 
even when they are single plants. Each plant is represented by the initial 
letters of the name. Signs may be used (Thornber, 1901:29). but they make 
charts difficult to grasp, and have the great handicap of differing for every 
investigator. The first letter of the generic name is used if no other genus 
found in the same quadrat or series of quadrats begins with the same letter. 
If two or more genera have the same initial, e. g., Agropyrum, Allium, and 
Anemone, the most abundant one is designated by a, and the others by the 
first two letters, as al, an. When a similarity in names would require three 
or more letters, e. g., Androsace, Anemone, and Antennaria, this is avoided by 
fixing upon an arbitrary abbreviation for one, viz, at. The number of stems 
from one base is often indicated by the use of an exponent, e. </.. a'. Seedlings 



190 



THE INVESTIGATION OF SUCCESSION 



are often distinguished by a line drawn horizontally through the letter, and 
plants in flower or fruit by a vertical line. In forest quadrats, seedlings are 
usually indicated by a small letter and mature individuals by a capital. In 
charting seasonal aspects, the rule is to indicate only the characteristic species, 
i. e., those that flower at the time concerned. 

The chief use of chart quadrats is for the comparison of different examples 
of the same community, or adjacent zones or stages of a sere. They are indis- 
pensable for the method of inference by which scattered stages are combined 
to show the course and sequence of a sere. Since permanent quadrats give 
all the values of simple chart quadrats, and many others besides, the chart 
quadrat should be used only when a single visit to a region makes the perma- 
nent quadrat unavailable. 




1 " ' i" " 3 4 5 7 8 9 10 

Fig. 4. — Quadrat showing reproduction in a complete burn, Long's Peak, Colorado. 



Permanent quadrat. — In exact successional research it is imperative to be 
able to follow the course of development in detail from year to year, and espe- 
cially from one minor sun-spot cycle to another. This is possible only by 
means of quadrats whose location and limits are fixed so that they can be 
relocated and charted from season to season, year to year, or from one period 
to another. These are termed permanent quadrats (Clements, 1905:170; 
1907:208), since they make it possible to secure a complete record of all 
successional changes in the area studied. Naturally, they are always recorded 



THE QUADRAT METHOD 



191 



in the form of charts, though they may serve merely for an annual census of 
one or more species when this alone is desired. Permanent quadrats may be 
modified for various purposes, but they fall more or less completely into two 
groups, viz, permanent quadrats proper and denuded quadrats. The former 
are designed to reveal and record the changes shown by the different stages 
or associes of a sere ; they make it possible to follow the course from one stage 
to the next. Denuded quadrats enable the student to reestablish earlier con- 
ditions in the area by removing the reaction in some degree, and to produce 
lacking stages at will. It is not only possible to reestablish every usual stage, 
but also to prepare a larger number of areas with minute reaction differences 
and thus obtain an analysis of associes possible in no other way. Most 
important of all exact methods is the combination of permanent and denuded 




Fig. 5. — Quadrat showing seedlings of lodgepole pine in a Vaccinium cover, 
Long's Peak, Colorado. 



quadrats into pairs, throughout a series of serai communities and zones, as 
is indicated later. 

The permanent quadrat is staked out and charted in the manner already 
described for the chart quadrat. The selection of areas requires greater care, 
however, if they are to yield the best evidence of development. Like all 
accurate work, quadrating is slow, and hence the most important task is to 
secure the maximum results with the minimum number of quadrats. As a 



192 THE INVESTIGATION OF SUCCESSION 

rule, this means at least one quadrat in each consocies, with additional ones for 
important soeies. As soon as the quadrat has been mapped and photographed, 
a labeled stake bearing the number and the date is driven at the upper left- 
hand corner, and a smaller one is placed at the opposite corner to facilitate 
the accurate setting of the tapes in later observations. It is also essential to 
select and record definite landmarks with care, in order that the location 
may be readily found again. In forest or scrub this is readily secured by blaz- 
ing, but in grassland it is necessary to erect an artificial landmark, or to resort 
to compass and pacing. 

At successive readings of a permanent quadrat, the tapes are placed in exact 
position by means of the stakes, and chart and photograph are made in the 
usual manner. To facilitate the study of the charts, four successive readings 
are recorded on the same sheet, thus greatly reducing the mechanical labor 
involved in comparing separate sheets. The same advantage is secured where 
the quadrat is used to show the variations from aspect to aspect of the same 
year. While the permanent quadrat reveals the actual changes in composition 
and structure which occur in the course of succession, a large part of its value 
is lost unless it is made a station for measuring the physical factors involved 
in ecesis, competition, and reaction. 

Denuded quadrat. — A denuded quadrat (Clements, 1905:173; 1907:209) 
is a permanent quadrat from which the plant covering has been removed, after 
having been charted and photographed. Quadrats in bare areas, both prim- 
ary and secondary, are essentially similar, but they differ in the impossibility 
of charting the original cover and of controlling the kind and degree of 
denudation. The denuded quadrat is especially adapted to the analytical 
study of ecesis and competition in relation to reaction. While denuding is an 
invaluable aid to the study of succession, it must be recognized that permanent 
quadrats register the exact course of development, while denuded ones make 
possible more definite analysis, and throw light upon stages not now available. 

A quadrat which is to be denuded is first mapped, photographed, and 
labeled as for a permanent quadrat. The vegetation is then destroyed by 
removal, burning, flooding, or in some other manner. The kind and degree of 
denudation will be determined by the evidence sought. If it is to throw light 
upon an area in which denudation has affected the surface alone, the aerial 
parts may be removed by paring the surface with a spade, or by burning. 
To trace the effect of a more profound disturbance upon the reaction, the soil 
may be removed to varying depths, it may be dug up and the underground 
parts completely removed, or a sterile soil may be used to replace it. For 
obvious reasons, denuded quadrats are most valuable when used in connection 
with permanent quadrats, as indicated below. 

Quadrat series and sequences. — In following the sequence of stages, the 
most valuable method is to use paired quadrats in each associes or consocies. 
Each pair consists of two permanent quadrats located side by side. After 
being mapped, one of them is denuded in the manner desired, and the two are 
then charted annually on the same sheet. If a battery of instruments for 
recording light, humidity, and temperature is located in the area, and the soil 
factors are determined for the two quadrats, a complete and accurate picture 
of succession is obtained. The permanent quadrats link the stages together as 
they occur ; they fix the attention upon the process rather than upon the more 



THE TRANSECT 193 

striking results. The denuded quadrats permit the ready analysis of the 
basic processes of migration, ecesis, competition and reaction, and the instru- 
ments furnish the necessary data as to the controlling physical factors. 

If the analysis of processes and habitat is to be as thoroughgoing as possible, 
it is necessary to use a sequence of denuded quadrats. A time sequence is 
established by denuding one quadrat each year, each new area being separated 
by a space of a meter or so from the preceding one, so that invasion may occur 
from all sides. In this way it is possible to reproduce a complete series of 
stages, and to have them in close juxtaposition for comparative study. A 
quadrat sequence in space may also be used for the analysis of reaction, by 
denuding a series of areas in the same community in different ways or to 
different depths. 

Various quadrats. — With more or less modification, the quadrat method 
may be applied to all plant communities, even in the most extreme areas. In 
fact, some of its most striking results are obtained with the pioneer com- 
munities in water and on rock. Chart quadrats of aquatic consocies are 
readily made, though permanent and denuded ones present obvious difficulties. 
Lichen and moss quadrats, on the other hand, are easily made permanent, or 
are readily denuded. Those under observation in the Rocky Mountains 
promise most interesting results, though the changes are necessarily slow. 
Subquadrats of parasitic and and saprophytic communities on bark, fallen 
trunks, and on the ground may likewise be made permanent, though the 
results are of secondary importance. Moreover, it seems probable that the use 
of soil quadrats will open a new field of study in enabling us to analyze the 
root relations of communities with much greater accuracy. 

The transect— The transect (Clements, 1905:176; 1907:210) is essentially 
an elongated quadrat. In its simplest form it is merely a line through a 
community or series of communities, on which are indicated the individuals 
of the species met with. The value of such a line transect lies chiefly in the 
fact that it reveals the larger changes of population, and hence serves as a 
ready means of delimiting ecotones. The belt transect consists of a belt of 
varying width, from a decimeter to several meters or more. It corresponds 
to the chart quadrat, and likewise gives rise to permanent and denuded 
transects. 

A line transect may be made by pacing an area and noting the species and 
individuals encountered. The usual method is to run a transect by means 
of tapes. In the case of belt transects, two tapes are employed to mark out a 
strip of the width desired. In grassland and undergrowth, a transect 2 deci- 
meters wide is most convenient, while in forest 1 or 2 meters wide is most 
satisfactory when reproduction is to be taken into account. When the adult 
trees alone are considered, the strip may be of any width. The transect is 
located in the area to be studied by running the tapes from one landmark 
to another, fastening them here and there by means of quadrat stakes. When 
it runs through a diversified area, particularly in the case of transects 100 to 
1,000 m. long, the topography is determined by means of a transit, and the 
transect, when charted, is superimposed upon the topographic drawing. The 
charting of transects is done in the manner already indicated for quadrats. 
Because of their length, however, an assistant is almost indispensable in the 
work. To save the handling of many sheets, the practice is to record several 

li 



194 



THE INVESTIGATION OF SUCCESSION 



segments of a transect on the same sheet, when the width is not too great 
(% 6). 

It is obvious that the belt transect, like the chart quadrat, is greatly en- 
hanced in value if it is made permanent. The latter is readily done by means 
of careful blazing in woodland areas, but in grassland or in alternating wood- 
land and grassland areas the use of a transit or at least of a compass is neces- 



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Long's Peak, Colorado. 



THE BISECT 195 

sary. A label stake is driven at each end, on which is painted the number and 
date of the transect, as well as its direction and length. The position of the 
ecotones is indicated by smaller stakes bearing the transect number and the 
date when the ecotone was found at that point. These are left in place as they 
are added from year to year in order to indicate the shifting of the ecotone. 
This is the chief use of the transect, and serves well to illustrate the difference 
between the quadrat and transect. The permanent quadrat is intended to 
give the composition and structure of a typical or representative portion of a 
single community, and to enable one to follow its changes from year to year. 
A series of quadrats makes it possible to establish spatial comparisons between 
communities for any particular year, and has the incidental advantage of 
making it unnecessary to chart the intervening vegetation. This disadvan- 
tage of the transect, however, is more than offset by the unique values 
obtained by being able to trace the change in the typical structure and com- 
position of a community or zone through the transition features of each 
ecotone into the adjacent zones. It has already been emphasized that zones 
are serai stages, and that ecotones make it possible to discover how one stage 
passes into another, i. e., they are substages in essence. The transect alone 
makes it possible to follow in detail the change from one zone or associes to 
another through the ecotone, and hence is of the first importance in the inves- 
tigation of succession, especially by the method of inference. In the case 
of grassland and forest undergrowth, a combination of quadrat and transect 
would seem to constitute the best method, but this has not yet been tried. 
If the quadrats of a series through several zones were connected by narrower 
transects, the maximum information would be obtained with the minimum 
expenditure of time and labor. 

The denuded transect adds to the value of a permanent one by furnishing 
new stages for the analysis of each zone and ecotone ; hence the two should be 
employed together wherever time and opportunity permit the most intensive 
study. The simplest method is to chart a permanent transect of twice the 
width, and then to denude one-half the width throughout, Since it is the 
colonization on the bare strip that is of importance, a permanent transect may 
be made in the usual way, and then a strip of equal width alongside of it 
denuded without being charted. 

The bisect. — The layer transect (Clements, 1905:180) is used to show the 
vertical relations of species in a layered community. Its value in succession 
lies chiefly in recording the successive disappearance of layers as the climax is 
reached. It has further value in tracing the beginnings of layering as com- 
petition passes into the dominance of medial stages. In all of these cases, root 
relations play an important and often a controlling part. Hence they consti- 
tute an essential portion of the record, and it is proposed to indicate the 
vertical and lateral relations of individuals by means of a cross-section show- 
ing both shoots and roots in their normal position. Such a cross-section may 
be termed a bisect (figs. 7, 8). In a purely diagrammatic form it has fre- 
quently been used to show the relations of aquatic and swamp plants, but as a 
means of showing the exact relations of layers, especially in the soil, it was 
first employed by Yapp (1909:288) and Shantz (1911:51). The latter used 
it primarily to illustrate the reaction of root-layers upon water penetration, 
and the consequent effect upon the course of succession. There is no question 



196 



THE INVESTIGATION OF SUCCESSION 



that investigations of this sort must become increasingly frequent in the study 
of development, and that the bisect will become a regular method of investi- 
gation and record (c/. Weaver, 1915, 1916). 




Fig. 7. — Bisect of sandhills mixed association in eastern Colo- 
rado, a, Calamovilfa longifolia ; b, Artemisia filifolia; c, An- 
dropogon scoparius; d, A. hallii; e, Ipomoea leptophylla; 
f, Aristida purpurea; g. Bouteloua hirsuta. After Shantz. 



I ft r 




Ift. L 

Fig. 8. — Bisect of the Bulbilis-Bouteloua-poion in eastern Colo- 
rado, a, Bouteloua gracilis; b, Bulbilis dactyloides. After 
Shantz. 



The migration circle. — The migration circle (Clements, 1905 .482 ;1907 :212), 
or migrarc, is designed to make possible the exact analysis of migration, es- 
pecially without reference to ecesis. Practically all studies of migration have 



THE MIGRATION CIRCLE 



197 



been based upon the establishment of individuals, and there has been almost 
no attempt to determine the role of migration itself in succession, as one of the 
two processes concerned in invasion. A few studies have been made of the 
kind and number of migrules brought by long-distance carriage, such as that 
of birds, but these have little or no effect upon succession. Local, and usually 
mass, migration is the chief factor in the latter, and such movement permits 
of fairly accurate measurement, even though ecesis is not taken into account. 





( 


, 






a 














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ble — ■ 




V 




!/ 








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22 


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23 


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25 










26 




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27 


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29 


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3) 










32 


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35 








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v v 




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41 






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42 




d 
d v 


d 





4 dem. — wide 
Scale 1cm.= 1dm. 



1/= yearlings 

s = seedlings (older than y) 

d = dead trees 



Seed'lingsand yearlings are 

without exception restricted 

to free soil areas between. sod 

patches or mossy areas. In 

holes or shallow depressions 

where there is an absence of 

soil cover the yearlings are 

most abundant. 
S.w. 



N.E. 



Fio. 9. — Transect of a migration arc, Uncompahgre Plateau, Colorado. 



198 THE INVESTIGATION OF SUCCESSION 

A migration circle or arc may be located with reference to an individual, 
a community or a distinct ecotone, such as the edge of a forest. In the case 
of an individual, or a small family or colony, a circle, or better, a series of 
concentric circles, is marked out. This may be done by carrying a radius 
around the object, or, better still, where the study is to take some time, by 
means of concentric circles made by using a tennis-court marker. When the 
migration is in a single general direction, as at a forest edge, concentric arcs 
or parallel lines are used, as the size of the community demands. The deter- 
mination of migration alone demands the most minute study, and hence it is 
difficult to carry it throughout a season. In the case of wind-borne germules, 
it can best be determined after times of high wind, or, still better, at times of 
varying velocities which are measured. The detection of seeds and fruits 
in vegetation is so time-consuming, even when it is possible, that the study of 
actual movement can best be made upon snow surfaces or upon the bare 
ground. In the case of individual plants, especially trees, a denuded area in 
the direction of usual movement is the most satisfactory. When the study of 
migration receives the detailed attention which its importance warrants, it is 
probable that quadrats artificially prepared to catch and retain the migrules 
brought to them will be placed at definite intervals in the direction of migra- 
tion. At present, the measurement of effective migration in terms of ecesis 
furnishes the most convenient method. This has been used with signal success 
in investigating the invasion of "natural parks" by Picea and Abies in the 
Uncompahgre Plateau of Colorado (fig. 9) and recently by Hofmann (1916) 
in studying secondary succession in the Pacific coniferous climax. 

METHODS OF MAPPING. 

Methods. — Schroter (1910:127) has discussed at length the methods of 
ecological cartography. Such macrographic methods are not considered here, 
as they have little or no bearing on the intensive study of succession. This 
is due to the fact that succession has usually not been taken into account at 
all, though it is obvious that the areas mapped may be readily distinguished 
as climax and developmental. The methods indicated below are micrographic 
in the sense of Nilsson in that they deal with individual plants or communities. 
They may be distinguished as (1) extensions of the quadrat method, (2) 
ecotone and community mapping, and (3) combined quadrat and map method. 
The first is illustrated by the method of squares and the gridiron method of 
Oliver and Tansley (1904:228, Tansley 1904:200). The gridiron method 
appears to be very similar to the use of perquadrats of 25 feet, but differs in 
that the contours and the outlines of communities are indicated rather than 
the individuals. The method of squares employs areas 100 feet square, which 
are used to cover continuously the entire area to be mapped. Physical fea- 
tures, contours, and the boundaries of communities are alone shown, though 
the gridiron method may be used to furnish greater detail in various areas. 

Community charts and ecotone maps. — Community charts (Thornber, 
1901:126) resemble closely the gridiron maps of Oliver and Tansley. They 
are made by means of pacing or by tapes. They may be employed to map in 
some detail the dense vegetation of an associes or association, but their greatest 
value lies in indicating the position and growth of families and colonies in 



METHODS OF MAPPING 199 

bare areas. Community charts combine quadrat and map methods, in that 
they deal chiefly with the outline of units, but they are on such a scale as to 
show much of the detail as to composition. 

Ecotone maps (Clements, 1905:181) have to do with the relations of zones 
and alternes. Because of the essential relation of the latter to succession, such 
maps furnish a graphic summary of the course of development, either actual 
or potential. In the one case, the map shows the zones or stages of an actual 
sere on a small scale, as in the case of ponds and streams. In the other, the 
zoned climax associations of an entire region are shown on a large scale in the 
potential sequence of the clisere. In either case, the map is constructed by 
locating and tracing the ecotones between the various zones, usually with the 
topographic features indicated in so far as they have a bearing upon develop- 
ment. 

Survey maps. — In the methods of vegetation survey developed by the 
Botanical Survey of Minnesota, an endeavor has been made to combine the 
advantages of topographic and ecotone maps with those of quadrats. From 
the nature of American subdivisions, the unit is the township, consisting of 36 
square miles or sections. Each section is divided into four quarter sections a 
half mile square, and each of these into "forties" a fourth of a mile square. 
The mapping unit is the "forty." This is mapped on a square decimeter of 
cross-section paper, the four maps for each quarter section being placed in 
their proper relation upon one sheet. The scale is approximately 1 to 4,000. 
Topographic and artificial features and the ecotones of communities are 
mapped in detail. Cultural as well as natural communities are indicated, 
while at least one quadrat or transect is charted for each ' ' forty, ' ' the number 
depending upon the differentiation of the vegetation in it. Instrument read- 
ings are taken in the quadrats or at the ecotones, and photographs are made to 
accompany the charts throughout. As a consequence, a complete record is 
obtained of topography and the structure and development of vegetation, with 
some idea of the physical factors involved as well. When supplemented by 
intensive studies of prisere and subsere in the different climaxes, a complete 
picture of the vegetation is obtained. The application of the results of such a 
survey also becomes a matter of prime importance to forestry and grazing 
and to the agriculture of new or neglected regions. 

Climax maps.— The general treatment of vegetation as static has resulted 
in the production of many maps in which no distinction is drawn between 
climax and developmental communities. From the nature and extent of 
climax formations, vegetation maps of regions and continents have been con- 
cerned with them primarily, but with little or no recognition of basic develop- 
mental relations, such as that of the clisere. Vegetation maps have been 
constructed from many sources of diverse value, and can only be regarded as 
provisional to a large degree. The existence of a great climax vegetation is 
so patent that its general area can readily be indicated, but its definite relation 
to other climaxes, its exact boundaries, and especially the problems of such 
transition areas as prairie, chaparral, and mixed forest can only be settled by 
intensive studies. Hence the construction of reliable climax maps must follow 
the investigation of developmental relations and the accurate tracing of great 
ecotones rather than precede them, as has usually been the case. However, 
it is clear that it is necessary to construct such maps from time to time as our 



200 THE INVESTIGATION OF SUCCESSION 

knowledge grows, in order that they may serve as working bases for further 
refinement. For the present the methods of cartography already in use for 
macrographic maps will suffice, but it seems clear that these must be largely 
worked over when maps come to be used to show primary developmental rela- 
tions. 

INSTRUMENTAL METHODS. 

General considerations. — While there has been a notable advance in the 
use of instruments since the appearance of "Research Methods in Ecology" 
(1905), the instrumental study of vegetation is still far from the rule. This is 
strikingly true of succession, for the additional reason that developmental 
studies themselves are still exceptional. As indicated in the discussion of 
reactions, the use of instruments in studying successional processes was begun 
in America more than a decade ago, but it is only during the last two or three 
years that instrumentation has become a general procedure in this country 
and in England. Elsewhere, even in Scandinavia, where developmental 
studies have long been the rule, the instrumental study of successional pro- 
cesses is still infrequent. There are evidences, however, that this condition 
is disappearing, and we can look forward confidently to a time when succession 
will become the basic method of vegetation research, and when it will use 
instrument and quadrat as its most indispensable tools. 

The chief use of instruments so far has been in attempting a complete or 
partial analysis of the habitat. All careful work of this sort furnishes data 
for succession, but much of it is difficult of application or interpretation. As 
a consequence, the use of instruments in developmental study must be directed 
to the critical processes in succession. These are reaction, ecesis, and com- 
petition. The first of these is clearly the most important, because of its 
control of the movement of successive populations, but its effect in plant 
terms is measured by ecesis and competition also. The critical effect of reac- 
tion is felt at the time of germination, and when competition between the 
mixed populations of a mictium is passing into the dominance of the next 
stage. Hence the measurement of reaction has its greatest value when it is 
directed to these two points. It must also be recognized that reaction is itself 
a complex process, in which all of the factors of the habitat may be concerned. 
Here, again, it is essential to keep in the main path, and to concentrate upon 
the primary reactions which direct the actual sequence of stages. As has been 
shown in Chapter V, the primary reactions are upon water and light, and 
upon the stability of the soil, though the latter can perhaps best be measured 
in terms of humus and water-content. In some cases, reaction upon nutrient 
content plays a primary role, as perhaps also that upon water by which it 
becomes acid. It is clear that these two reactions may also be intimately 
bound up with each other. In initial and medial stages the edaphic reactions 
are controlling, but in the final stages of scrub and forest formations the light 
reaction is decisive. At the same time the water reaction can not be ignored, 
as Fricke has demonstrated (p. 93). The local climatic reactions of forests 
may ultimately prove of much importance, but they would seem to play only 
a subordinate part in the development of a particular sere. 

The instrumental study of succession must be made chiefly in the reaction- 
level. This is the level which is bisected by the surface and is characterized 



INSTRUMENTAL METHODS 201 

by the maximum effect of reaction. It is the level also in which the critical 
decisions as to ecesis and competition are reached. The measurement of 
reaction at other levels is not without value, but it is rarely of primary impor- 
tance. Since the critical period for each species is usually the seedling stage, 
it indicates that the depth of the reaction-level above and below the surface 
is only a few inches, or at most a foot or so. This greatly narrows the field 
of measurement, and makes the application of the results much easier. The 
most critical area of all is where the reaction-level of one community meets 
that of another, i. e., the ecotone. This is strikingly illustrated by the re- 
ciprocal behavior of the seedlings of both communities at the ecotone between 
grassland and forest. It is in such areas that reactions can be best determined 
and their influence measured. 

Measurement of reactions. — The methods of instrumental investigation 
(Clements, 1905:20; 1907:7, 73) are now so numerous and detailed that no 
adequate account of them can be attempted here. It must suffice to point out 
the general method of attack and to emphasize the necessity of such study 
for the understanding of succession. At the outset, it must be recognized 
that general measurements, such as are made in the usual meteorological 
observations, are of little or no value. This is true to some extent also of the 
data obtained by ecograph batteries in various habitats. These bear some 
relation to the conditions in actual control of ecesis and competition, but the 
direct attack must be made upon these conditions themselves, since they 
characterize the reaction-level. Thus, while the experienced investigator may 
find it possible to interpret and apply general factor data, one can expect 
to obtain little light upon succession unless the instrumental study is con- 
centrated upon the factors in primary control. This means that water and 
light reactions must be given the first place, though it is certain that water 
reactions in particular must ultimately be carried further back into the plexus 
of intricate cause-and-effect relations found in the soil. Moreover, in dealing 
with water and light, it must be borne in mind that the reaction may affect 
the quality as well as the quantity, and that humidity as well as water-content 
must be considered. Finally, it should be recognized that, while instruments 
furnish the readiest means of measurement, the use of standard plants for 
determining the effect of each reaction brings us much nearer to the explana- 
tion sought. 

Measurement of water reactions. — The reaction of a community upon 
water may affect the amount of holard and echard, or the degree of humidity. 
It may change the nature of the water-content by modifying the nutrient 
content, by making it acid or decreasing its acidity, or by decreasing its alka- 
linity. So far as is known the alkalinity of the soil can not be increased by the 
accumulation of plant remains in it, except by artificial means. The develop- 
ment of instruments and instrumental methods for the study of water-content 
and humidity has gone so far that even a brief mention of them is impossible 
in the scope of the present discussion. The great majority of them have not 
been developed with reference to succession, and hence the number which 
require mention is small. Those of the first importance for the accurate field 
study of reaction are: (1) determination of the holard in the reaction-level 
of the soil, and especially at the germination level; (2) determination of chre- 
sard and echard at the depth of various roots in the reaction-level, since the 



202 THE INVESTIGATION OF SUCCESSION 

addition of humus, and often the abstraction of water also, decreases from the 
surface downward; (3) measurement of humidity and evaporation in the chief 
reaction level of the air, and especially at the soil-surface, where the effect 
upon the seedling is critical; (4) determination of the degree of acidity of soil- 
water at different depths; (5) determination of the degree of alkalinity at 
different depths. 

Methods of determining the holard are so numerous and so simple as to 
need little comment. From the standpoint of succession, however, it is imper- 
ative to determine the holard at levels marked by the root-layers, and espe- 
cially in the soil-layer occupied by the roots of dominants. But, while it is an 
easy matter to measure the reaction in terms of increased water-content, the 
successional significance of this increase can be determined only by ascertain- 
ing the amount of it available, i. e., the chresard. Our knowledge of this avail- 
able water and of the water requirements of plants has greatly increased since 
the chresard was emphasized as the critical factor in vegetation (Clements, 
1905:30; 1907:9). In spite of the excellence of the work done under control 
conditions (Briggs and Shantz, 1912; Crump, 1913:96; 1913 2 :125), it seems 
certain that the role of the chresard in succession can only be determined 
under field conditions, owing to its variation at different soil-levels and per- 
haps with the conditions for transpiration. The measurement of evaporation 
has been so standardized by the porous-cup method in the work of Livingston 
(1910:111) and others, and by the open water method of Briggs and Belz 
(1910:17) that there is little left to be desired. Headings of humidity, temper- 
ature, and wind have become unnecessary, except as they are required for the 
analysis of evaporation or for other purposes. In the case of evaporation, how- 
ever, while this gives a measure of reaction, it may not have a causal connec- 
tion with succession. This is apparently the case in the successive consocies of 
scrub and forest, where the evaporation decreases toward the climax, but the 
reaction in control of the sequence is that upon light. Moreover, evaporation 
measures fail to reckon with the compensating effect of water-content, and it 
seems inevitable that measures of transpiration be largely substituted for those 
of evaporation in the study of serai reactions. Considerable success has already 
been attained in selecting species and standardizing individuals for this pur- 
pose, and the method gives promise of universal application. Until we have 
a clearer notion of the actual effect of an acid holard, the present methods 
of determining the degree of acidity by means of litmus or phenolphthalein 
are fairly satisfactory. It seems increasingly certain, however, that the acid 
is merely a by-product of decomposition under a lack of oxygen, and that the 
absence of oxygen is the real factor. Experiments now under way seem to 
prove this, and hence to indicate that measurement of the primary reaction in 
acid soils must be directed toward the effect upon the oxygen content, i. e., 
upon aeration. The determination of the alkalinity of the soil solution has 
been so thoroughly worked out by Briggs (1899) by means of electric resist- 
ance apparatus that it seems to leave nothing more to be desired. 

Measurement of light reactions. — Since the pioneer work of Wiesner 
(1895) in measuring light intensity, a number of methods have been devised 
to measure light values (Clements, 1905 :48; 1907 :72; Zon and Graves, 1911). 
Most of these have had to do with light intensity, but the spectro-photometers 
of Zederbauer (1907) and Knuchel (1914) have been devised for the purpose 



GROWTH METHODS 203 

of determining the quality of forest light. Most instruments for measuring 
light intensity have been based upon the use of photographic paper. Theo- 
rectically this is unsatisfactory, because only the blue-violet part of the ray 
is measured. Practically, however, the use of such photometers for more 
than 15 years has furnished convincing evidence that it is a very satisfactory 
method of measuring the effect of light in the structure and development of 
communities and the adaptation of species. In the endeavor to organize the 
whole field of light instrumentation, the writer has designed and used with 
steadily increasing efficiency the following series of photometers: (1) simple 
photometer; (2) stop-watch photometer; (3) water photometer; (4) scia- 
graph, or recording photometer; (5) spectro-photometer. The construction 
and operation of these are described in detail in a forthcoming paper. In 
addition, a further effort is being made to develop a method by which stand- 
ardized plants are employed for determining the amount of photosynthate in 
different serai stages. 

GROWTH METHODS. 

Ring-counts. — In determining the successional relations, and especially 
the sequence of woody dominants and subdominants, determinations of the 
respective ages by counts of the annual rings is of the first importance. This 
is especially true of supposed cases of degeneration of forest or its conversion 
into scrub, heath, or grassland. There is no substitute for this method, except 
the all but impossible one of tracing the course of development throughout, 
which would require more than a life-time. It is for this reason that all 
reported cases of natural degeneration or conversion have been called in ques- 
tion, as well as many of those where the operation of artificial factors is slow. 
In none of these have the exact methods of ring-counts and quadrats been 
employed, and in consequence the conclusions reached can only be regarded as 
working hypotheses. In the more minute studies of sequences and of dates 
it has been found possible to determine the ages of perennial herbs, by the 
rings as well as by the joints of their rhizomes or other underground parts. 
This is of particular value in the study of colonization after fire or other 
denuding forces. Ring-counts can be used to the greatest advantage in ascer- 
taining the relations of dominants in mictia and in ecotones, but they are also 
indispensable in determining the serai significance of relicts. In fact, the 
recognition of relicts often depends wholly upon the determination of respec- 
tive ages. In reproduction, especially under competition, and particularly 
where forest or scrub is in contact with grassland, the ages of the invading 
trees or shrubs at various distances from their community is indispensable to 
a knowledge of the present success and the future outcome of the invasion. 

A detailed account of methods of counting rings seems unnecessary because 
of the general simplicity of the problem. Certain precautions are necessary, 
however, as well as great care in the actual process of counting the rings 
(Clements, 1907, 1910; Douglass, 1909, 1913, 1914; Huntington, 1914V 
Stump-counts are desirable as a rule, but in many cases the increment borers 
of foresters can be used to advantage. Fortunately, lumbering and clearing 
usually furnish the necessary stumps, though the intensive study of succession 
over many areas can only be carried on by the constant use of ax and saw. 



204 THE INVESTIGATION OF SUCCESSION 

Burn-scars. — The method of using ring-counts and the scars left by fire 
upon trees and shrubs for determining successional changes, as well as the 
dates of their occurrence, is described in the following extract (Clements, 
1910:9). 

' ' The basic method of reconstructing a burn has been to determine the ages 
of the oldest plants which have come in since the fire, applied both to the trees 
and to the shrubs and perennial herbs of each type. It takes account of dead 
trees and shrubs, standing and fallen, in addition to the living ones. The 
method of fire-scars is of equal importance, though often less available. 
Where the same area has been burned over two or more times this method is 
of unique value, for it is not unusual to find double and even triple scars. 
The nature, position, and extent of fire scars are of equal importance. Any 
evidence left by a fire upon a woody plant is regarded as a scar. Hence it is 
possible to distinguish top scars, trunk scars, and base scars with respect to 
position, and bark scars and wood scars with respect to depth. Heal scars 
and hidden scars occur on living trees, while white scars and cinder scars are 
found on dead or drying trees. In addition to ages and scars, the observation 
of soil layers is often of great help. The presence or absence of a cinder layer 
or of cinder pockets, or of an organic layer or cover often goes far to check or 
confirm the evidence drawn from ages or scars. 

' ' The evidence of age drawn from annual rings is usually so clear and deci- 
sive as to be beyond question. Occasionally with seedlings, and often in the 
case of suppressed trees, it is impossible to make an absolute count of rings, 
even by means of microscopical sections. In practically all such cases an 
examination of other individuals is conclusive. 

"A distinction between fire-scars and scars due to other causes is sometimes 
made with the greatest difficulty, and in rare cases is altogether impossible. 
In most cases, however, it is possible to recognize a fire scar with certainty. 
In actual practice the method was to require evidence of charring wherever 
the age of the scar did not check with that of neighboring scars. Chance 
scarring by lightning or by a camp fire, often in unexpected places, is of suffi- 
cient frequence to explain the departure of any charred scar from the normal. 
The position of a scar often serves to determine whether it belongs to a par- 
ticular fire or is a mere chance scar. Heal scars abound at the edges of a burn, 
and consequently those caused by the same fire occur on the same side, namely, 
that from which the fire came. Occasional exceptions arise where a ground 
fire has unexpectedly worked to the surface, but these are nearly always 
determined by a careful scrutiny. The nature of fires and their severity is 
indicated, as a rule, by the depth of scars, and the predominance of bark scars 
or wood scars is used to determine the relative order of two or more successive 
fires. Scars from successive fires are often united in double or triple scars on 
either dead or living trees, and these give the best of all evidence upon the 
succession of fires and the burn forests which follow them. In using the 
depth or nature of scars as a guide the fact was considered that forests regu- 
larly contain dead standing trees, some of which may have lost their bark. 
It is evident that the same fire would cause at least three different kinds of 
scars in such stands ; that is, heal scars, usually basal, on the surviving trees ; 
bark scars on the living trees killed by the fire ; and wood scars on the dead 
trees. The wood scars would, moreover, be cinder scars wherever the bark 
had fallen off before the fire had occurred. 

"Finally, the data obtained from fire scars were checked by a count of the 
annual rings formed since the scar was made. The most careful use of the 
evidence from fire scars and from annual rings can not eliminate the possibil- 



GROWTH METHODS 205 

ity of an error of one year in determining the date of a fire. This is because 
the time of the growing season at which a fire occurs determines whether 
growth or germination may begin that year. "With scars and root suckers 
on trees which remain alive, it is probable that growth begins the year of the 
fire, unless it occurred in the fall or early winter. On the other hand, it is 
equally probable that seeds remain dormant until the following year, unless 
the fire occurs in spring or early summer. The majority of fires occur after 
midsummer. If the growing season is not over, scars and root suckers will 
show one more ring than the pines and perennial herbs which appear the 
next year. If it is after growth ceases, scars, root sprouts, pine seedlings, and 
perennials will agree in the number of rings. In most of the burns studied, 
scars formed the first ring the year of the fire, while the pine seed did not 
germinate until the next spring. In the burn of 1905 the aspen-root sprouts 
followed the fire immediately, but in the burns of 1901 and 1878 aspens and 
pines appeared together the year after the fire. Therefore the following simple 
rule was used to determine the year of a fire : Subtract the number of rings of 
a scar, or the number of rings plus one of a seedling or tree, from the year in 
which the count is made. This rule assumes that the trunk is cut sufficiently 
low to show the first year's growth. With lodgepole it was necessary to cut 
the trunk at the surface of the ground, or on slopes, below the surface." 



; 



PLANT INDICATORS 

THE RELATION OF PLANT COMMUNITIES 
TO PROCESS AND PRACTICE 

An Abridgment of Publication No. 290 
CARNEGIE INSTITUTION OF WASHINGTON 

1920 



XI. CONCEPT AND HISTORY. 

The practical aspect. — Every plant is a measure of the conditions under 
which it grows. To this extent it is an index of soil and climate, and conse- 
quently an indicator of the behavior of other plants and of animals in the 
same spot. A vague recognition of the relation between plants and soil must 
have marked the very beginnings of agriculture. In a general way it has 
played its part in the colonization of new countries and the spread of cultiva- 
tion into new areas, but the use of indicator plants in actual practice has 
remained slight. It is obviously of greatest importance in newly settled 
regions. However, it is in just these regions that experience is lacking and 
correlation correspondingly difficult. In fact the pioneer is often misled by 
his endeavor to transfer the experience gained in his former home to a new 
and different region. Differences of vegetation and climate, and often of soil 
as well, make a wholly new complex of relations. As a consequence, the 
settler is very apt to go astray in reaching conclusions as to the significance 
of a particular plant. As the country becomes more settled, experience 
accumulates and makes it increasingly possible to recognize helpful correla- 
tions. But this period usually passes too quickly to establish a procedure 
before the native plants have disappeared, except from roadsides, meadows, 
and pastures. The manner and degree of utilization of natural meadows and 
pastures are clearly indicated by the plants in them. Yet it is exceptional 
that these indicators are recognized and made use of by the farmer. 

The scientific aspect. — On the scientific side, the concept of indicators 
could hardly be expected to emerge until plant physiology had made a begin- 
ning. Looking backward, one discerns something of this idea in the studies of 
vegetational changes by King (1685:950), Degner (1729), Buffon (1742:234, 
237), and Biberg (1749:6, 27). It is likewise suggested in the description of 
stations by Linne (1751:265) and especially by Hedenberg (1754:73). The 
basic correlations were made definite by De Luc (1806: Plant Succession, 10) 
in his studies of succession in peat-bogs and by Schouw (1823 :157, 166) in the 
classification of plants by habitats. The idea is more or less in evidence in the 
long series of observations and discussions relating to the chemical theory of 
the influence of soils. The chief proponents of the chemical theory were Unger 
(1836), Sendtner (1854), Naegeli (1865), Fliche and Grandeau (1873), 
Bonnier (1879), Contejean (1881), Hilgard (1888, 1906), and Schimper 
(1898, 1903). The founder of the physical theory was Thurmann (1849), 
though his views necessarily placed his results in more or less harmony with 
the water-content classification of Schouw. The century-old controversy over 
the chemical theory has centered around the question of the importance of 
lime in the soil. Though the broadening of ecological research has thrown 
this question more and more into the background, there is still anything but 
unanimity of opinion concerning it. While it is felt that the probiem can be 
solved only by more comprehensive and thoroughgoing experimentation than 
it has yet received, the several divergent views are later considered briefly for 
the sake of a clearer appreciation of existing opinion. Finally, the many 

2 119 



210 CONCEPT AND HISTORY 

studies of foresters upon the tolerance of trees to shade had large elements 
of indicator value, but these were never brought together into a system. 

Studies of the relation of plants to soil were based upon the response of the 
individual or species. The first serious attempt to organize these into a sys- 
tem of indicator plants was made by Hilgard (I860, 1906). In a similarly 
virgin region, Bessey (1891, 1901) also recognized the indicator value of 
native plants, and especially vegetation, for the proper development of agri- 
culture. His ideas of the practical value of vegetational studies stimulated 
the development of ecology as recorded in the " Phy togeography of Nebraska" 
(Pound and Clements, 1898, 1900) and the "Development and Structure of 
Vegetation" (Clements 1904:1). In the latter the need of quantitative studies 
of habitat and community and the importance of succession were first empha- 
sized, and these were made the basis of a definite quantitative system in 
"Research Methods in Ecology" (Clements, 1905). As a consequence, the 
way was prepared for the use by Shantz (1911) of the plant community as 
an indicator with particular reference to succession. In another direction, 
E. S. Clements (1905) made a searching investigation of the relation of leaf 
structure to different factors and habitats and laid the foundation for the use 
of habitat-forms and ecads as indicators. 

The development of the idea that plants are indicators of climate is more 
difficult to trace. Tournefort (1717) probably furnished the first recorded 
instance of the idea, when he pointed out that the slopes of Mount Ararat 
showed many species of southern Europe, while still higher appeared a flora 
similar to that of Sweden, and on the summit grew arctic plants such as those 
of Lapland. Perhaps the most important studies of climatic zones of vegeta- 
tion were those of Humboldt and Bonpland (1805:37), Kabsch (1855:303), 
Koppen (1884:215), Drude (1887:3), and Schimper (1898, 1903:209). In 
none of these is there a distinct recognition of the indicator concept. This is 
likewise true of the formulation of life zones and crop zones on the North 
American continent by Merriam (1898). His applications of the indicator 
idea are so numerous and definite, however, that he must be given the credit 
for organizing the first system of climatic indicators. As to the soil, Hilgard 
is to be regarded as the pioneer in recognizing the great possibilities of sys- 
tems of indicators and applying this on an adequate scale, and Shantz as the 
investigator who has placed the whole matter upon an adequate scientific 
basis. 

HISTORICAL. 

In a general account of the important steps in the spread of the indicator 
concept, it appears best to deal only with those studies in which the concept is 
either evident or actually stated. There are numerous books and papers on 
plant-geography, forestry, and agriculture, which have some general relation 
to the idea. Most of these have contributed nothing tangible or important 
and for the most part are ignored. A few are considered or mentioned in 
the proper special sections. Entire justice might demand consideration of 
the work of Bonnier, Fliche and Grandeau, and Contejean at this point, but 
for many reasons it has proved undesirable to treat these in detail. The 
following: accounts are of those researches in which the term indicator is 



HISTORICAL 211 

actually employed or in which the use of instrument, quadrat, or successional 
methods gives them distinct indicator objectives. 

AGRICULTURAL INDICATORS. 

Hilgard, 1860. — The following excerpt will serve to show that Hilgard 
was the first investigator to recognize clearly the importance of indicators in 
soil studies and to make actual use of them in determining the agricultural 
possibilities of new lands. A further account of his views and results is given 
on a later page. 

"Judging of land by its natural vegetation. The distinction just men- 
tioned, so far from being of merely theoretical value, is one of the highest 
practical importance. Agriculturists are accustomed to judge of the quality 
of lands by the natural vegetation which they find upon it; and they rarely 
direct their attention to anything but the forest trees. Yet these are, for the 
most part, indicative rather of what, in the agricultural sense is termed the 
subsoil, than that of the surface stratum usually turned by the plow, in the 
shallow tillage prevailing at present, which may be of a totally different 
character. 

"As a general thing, the forest growth when considered not only with 
regard to the kind (species), but also to the form and size of the trees, is a 
very safe guide in judging of the quality of land, and the systematic study of 
the subject in connection with analyses of soils, promises results of a highly 
practical importance, which it is intended to communicate more fully in a 
future report. But this criterion may not infrequently lead to grave mistakes 
unless a proper examination of the soil and subsoil be made at the same time. 

"These examples may suffice to show that while in the forest trees we 
possess trustworthy guides to a knowledge of the character of the material in 
which their roots are buried, it is quite essential to determine at the same 
time, by inspection, that it is the arable soil itself, and not merely the subsoil, 
which is thus characterized ; and we should especially make sure that the 
smaller plants, viz., the shrubs and perennials, corroborate the evidence of 
the trees. Annuals are less reliable in their indications because their develop- 
ment is to a greater extent influenced by the accidental circumstances of the 
seasons. ' ' 

Chamberlin, 1877. — Chamberlin shares with Hilgard the honor of being a 
pioneer in the use of native plants to indicate the agricultural possibilities of 
a region (1877:176). He deserves especial credit for being the first to recog- 
nize that the community was a better indicator than the species, and for classi- 
fying the vegetation of Wisconsin into communities with more or less definite 
indicator value. Several of Chamberlin 's associates on the Geological Survey 
of Wisconsin made more or less use of his system of indicators (Wooster, 
1882:146; King, 1882:614; Irving, 1880:89), though it unfortunately appears 
to have remained unknown to botanists, and consequently led to no further 
work in this field. 

"The most reliable natural indications of the agricultural capabilities of a 
district are to be found in its native vegetation. The natural flora may be 
regarded as the result of nature's experiments in crop raising through the 
thousands of years that have elapsed since the region became covered with 
vegetation. If we set aside the inherent nature of the several plants, the 
native vegetation may be regarded as a natural correlation of the combined 
agricultural influences of soil, climate, topography, drainage and underlying 
formations and their effect upon it. To determine the exact character of each 



212 CONCEPT AND HISTORY 

of these agencies independently is a work of no little difficulty ; and then to 
compare and combine their respective influences upon vegetation presents 
very great additional difficulty. But the experiments of nature furnish us in 
the native flora a practical correlation of them. The native vegetation there- 
fore merits careful consideration, none the less so because it is rapidly disap- 
pearing, and a record of it will be valuable historically. 

"It is rare in nature that a single plant occupies exclusively any consider- 
able territory, and in this respect there is an important difference between 
nature 's methods and those of man. The former raises mixed crops, the latter 
chiefly simple ones. But in nature, the mingling of plants is not miscellan- 
eous or fortuitous. They are not indiscriminately intermixed with each other 
without regard to their fitness to be companions, but occur in groups or com- 
munities, the members of which are adapted to each other and their common 
surroundings. It becomes then a question of much interest and of high 
practical importance to ascertain, within the region under consideration, 
what are the natural groupings of plants, and then what areas are occupied 
by the several groups, after which a comparison with the soils, geological 
formations, surface configuration, drainage and climatic influences, can not 
fail to be productive of valuable results. 

"The following natural groups are usually well marked, though of course 
they merge into each other where there is a gradual transition from the condi- 
tions favorable for one group to those advantageous to another. In some 
instances it is unquestionably true that other circumstances than natural 
adaptability control the association of these plants, and an effort has been 
made in the study of the region, to discern these cases and eliminate them 
from the results, so that the groups that are given here are believed to be 
natural associations of plants. Their distribution is held to show in what 
localities conditions peculiarly advantageous to them occur, and hence advan- 
tageous to those cultivated plants that require similar conditions." 

The author has used both class and group as synonyms of community, but 
the latter term is substituted in the following list for the sake of clearness : 
A. Upland vegetation. B. Marsh vegetation. 

(1) Herbaceous. 10. Grass and sedge community. 

1. Prairie community. 11. Heath community. 

(2) Arboreous. 12. Tamarac community. 

2. Oak community. 13. Arbor vitae community. 

3. Oak and maple community. 14. Spruce community. 

4. Maple community. C. Communities intermediate between 

5. Maple and beech community. upland and marsh. 

6. Hardwood and conifer com- 15. Black ash community, 
munity. 16. Yellow birch community. 

7. Pine community. 

8. Limestone ledge community. 

9. Comprehensive community. 

Merriam, 1898. — In "Life Zones and Crop Zones," Merriam summarized 
the experiential evidence as to the climatic indications for crop plants. This 
was arranged in relation to seven life zones based theoretically upon tempera- 
ture, but determined for the most part by the distribution of native plants 
and animals. As a pioneer attempt to organize a vast field, it deserves great 
credit, even though later studies have rendered his zonal classification of 
secondary value. The author's understanding of the nature and scope of 
climatic indicators is best shown by the following excerpts : 

' ' For ten years the Biological Survey has had small parties in the field 
traversing the public domain for the purpose of studying the geographic 
distribution of our native land animals and plants, and mapping the boun- 



HISTORICAL 213 

daries of the areas they inhabit. The present report is intended to explain 
the relations of this work to practical agriculture and to show the results 
thus far attained. 

"It was early learned that North America is divisible into seven trans- 
continental belts or life zones and a much larger number of minor areas or 
faunas, each characterized by particular associations of animals and plants. 
It was then suspected that these same zones and areas, up to the northern 
limit of profitable agriculture, are adapted to the needs of particular kinds 
or varieties of cultivated crops, and this has since been fully established. 
"When, therefore, the natural life zones and areas, seemingly of interest only 
to the naturalist, were found to be natural crop belts and areas, they became 
at once of the highest importance to the agriculturist. A map showing their 
position and boundaries accompanies this report, and lists of the more 
important crops of each belt and its principal subdivisions are here for the 
first time published. The matter relating to the native animals and plants 
has been reduced to a fragmentary outline for the reason that this branch 
of the subject is of comparatively little interest to the farmer and fruit- 
grower." (p. 7.) 

"The Biological Survey aims to define and map the natural agricultural 
belts of the United States, to ascertain what products of the soil can and what 
can not be grown successfully in each, to guide the farmer in the intelligent 
introduction of foreign crops, and to point out his friends and enemies among 
the native birds and mammals, thereby helping him to utilize the beneficial 
and ward off the harmful kinds." (p. 9.) 

"The farmers of the United States spend vast sums of money each year in 
trying to find out whether a particular fruit, vegetable, or cereal will or will 
not thrive in localities where it has not been tested. Most of these experi- 
ments result in disappointment and pecuniary loss. It makes little difference 
whether the crop experimented with comes from the remotest parts of the 
earth or from a neighboring State, the result is essentially the same, for the 
main cost is the labor of cultivation and the use of the land. If the crop 
happens to be one that requires a period of years for the test, the loss from 
its failure is proportionately great. 

"The cause of failure in the great majority of cases is climatic unfitness. 
The quantity, distribution or interrelation of heat and moisture may be at 
fault. Thus, while the total quantity of heat may be adequate, the moisture 
may be inadequate, or the moisture may be adequate and the heat inadequate, 
or the quantities of heat and moisture may be too great or too small with 
respect to one another or to the time of year, and so on. What the farmer 
wants to know is how to tell in advance whether the climatic conditions on his 
own farm are fit or unfit for the particular crop he has in view, and what 
crops he can raise with reasonable certainty. It requires no argument to 
show that the answers to these questions would be worth in the aggregate 
hundreds of thousands of dollars yearly to the American farmer. The Bio- 
logical Survey aims to furnish these answers." 

Life-zone surveys upon the basis laid down by Merriam have been made by 
Bailey for Texas (1905) and New Mexico (1913), and by Cary for Colorado 
(1911) and Wyoming (1917). Bobbins (1917) has made a somewhat similar 
study of the zonal relations in Colorado with reference to plants alone. Hall 
and Grinned (1919:37) have recently published comprehensive lists of plants 
and animals which are regarded as "life-zone indicators" for California. As 
with Merriam 's life zones, these are floristic and faunistic in character and 
hence do not necessarily correspond with community indicators. 



214 CONCEPT AND HISTORY 

Hilgard, 1906. — In summarizing his soil studies of more than 50 years, 
Hilgard formulated more fully and definitely his ideas of the indicator value 
of native vegetation. This account makes it clear that to Hilgard must be 
given the great credit of being the first to adequately realize the significance 
of indicators and to urge their inclusion in a basic agricultural method. 

"The importance of the natural relations of each soil to vegetation is obvi- 
ous, both from the theoretical and from the practical viewpoint. From the 
former, it is clear that the native vegetation represents, within the climatic 
limits of the regional flora, the result of a secular process of adaptation of 
plants to climates and soils, by natural selection and the survival of the fittest. 
The natural floras and silvas are thus the expression of secular, or rather 
millennial experience, which if rightly interpreted must convey to the culti- 
vator of the soil the same information that otherwise he must acquire by long 
and costly personal experience. 

' ' The general correctness of this axiom is almost self-evident ; it is explicitly 
recognized in the universal practice of settlers in new regions of selecting 
lands in accordance with the forest growth thereon ; it is even legally recog- 
nized by the valuation of lands upon the same basis for purposes of assess- 
ment, as is practiced in a number of States. 

"The accuracy with which experienced farmers judge of the quality of 
timbered lands by their forest growth has justly excited the wonder and envy 
of agricultural investigators, whose researches, based upon incomplete theo- 
retical assumptions, failed to convey to them any such practical insight. It 
was doubtless this state of the case that led a distinguished writer on agricul- 
ture to remark, nearly half a century ago, that he 'would rather trust an old 
farmer for his judgment of land than the best chemist alive.' 

"It is certainly true that mere physico-chemical analyses, unassisted by 
other data, will frequently lead to a wholly erroneous estimate of a soil's 
agricultural value, when applied to cultivated lands. But the matter assumes 
a very different aspect when, with the natural vegetation and the correspond- 
ing cultural experience as guides, we seek for the factors upon which the 
observed natural selection of plants depends, by the physical and chemical 
examination of the respective soils. It is further obvious that these factors 
being once known, we shall be justified in applying them to those cases in 
which the guiding mark of vegetation is absent, as the result of causes that 
have not materially altered the natural condition of the soil. (p. xix.) 

' ' It was from this standpoint that the writer originally undertook, in 1857, 
the detailed study of the physical and chemical composition of soils. It 
seemed to him 'incredible' that the well-defined and practically so important 
distinctions based on natural vegetation, everywhere recognized and contin- 
ually acted upon by farmers and settlers, should not be traceable to definite 
physical and chemical differences in the respective lands, by competent, com- 
prehensively trained scientific observers, whose field of vision should be broad 
enough to embrace concurrently the several points of view — geological, 
physical, chemical, and botanical — that must be conjointly considered in 
forming one's judgment of land. Such trained observers should not merely 
do as well as the 'untutored farmer,' but a great deal better." (p. 315.) 

This attitude toward plants and vegetation as indicators prevails through- 
out the book, and the subject is treated in considerable detail for the first 
time in Chapters XXIV to XXVI. These deal respectively with the recog- 
nition of the character of soils from their native vegetation, in Mississippi, 
and in the United States and Europe generally, and with the vegetation of 



HISTORICAL 215 

saline and alkali lands. While the author ascribes primary importance to the 
presence of lime, he does not fail to assign great value to water, especially 
in the West. He not only recognizes the indicator value of the presence of a 
particular species or group of species, but also takes into account the size, 
form, and development of the indicators. Significant tables and lists of 
indicators are given on pages 490, 497, 514-516, 518-519, and 536. 

Clements, 1910. — In 1908, the work of the Botanical Survey of Minnesota 
was reorganized upon an ecological basis, for the purpose of making a classi- 
fication and use survey of the lands of the State. The objectives of the survey 
were defined as follows (Clements, 1910:52) : 

"The first step in determining the final possibilities of Minnesota in plant 
production is to ascertain just what the conditions of soil and climate are 
from the standpoint of the plant. This must be determined separately for 
the two great groups of lands, those still unoccupied and those now in use. 
For the former, a knowledge of soil and climate and of the plant's relation to 
them is necessary to determine what primary crop, grain, forage, or forest is 
best. For the farms of the State, the best use is a matter of knowing the soil 
and climate differences of regions and fields, and of taking advantage of these 
in crop production. For the unoccupied lands of Minnesota, we need a classi- 
fication survey to determine the best use of different areas, to prevent the 
waste of human effort and happiness involved in trying to secure from the 
land what it can not give and yet to insure that the land will reach as quickly 
as possible its maximum permanent return. For occupied lands, the study 
and mapping of soil and climatic conditions would constitute a use survey of 
the greatest value in adjusting plant production to the conditions which 
control it. 

"The chief object of a classification survey is to group the unoccupied lands 
of the State as accurately as possible into three great divisions: (1) agricul- 
tural land, for crop production; (2) pasture land, for dairying and stock 
raising; (3) forest land, for lumbering, water regulation, and recreation 
parks. Such a division would be determined primarily by studies of soil and 
climate, necessarily supplemented by the evidence of native vegetation itself 
and of such cultivation as has been tried. The value of classification depends 
upon its accuracy, but the study of an area from these three standpoints 
neglects no source of evidence, and discloses practically all that can be 
learned of the possibilities." 

The survey method was based upon the instrumental and quadrat study of 
habitats and communities, cultural as well as natural. The main divisions 
were vegetation mapping, the determination of indicators, and the study of 
succession. Vegetation and physiography were recorded on maps in which 
each division of 40 acres was represented by a square decimeter. Quadrat 
and transect charts were made of typical communities in each section of the 
township, and determinations of physical factors in all charted quadrats. 
The indicator work was devoted to the recognition of indicator species and 
communities so closely dependent upon water-content, soil, acidity, or light 
that they could always be used as indicating a certain set of conditions. 
Especial attention was given to the correlation of indicators with crop plants 
and with the secondary successions in burns, cutovers, fallow fields, pastures. 
roadsides, etc. Four townships were mapped upon this basis in 1912, and a 
large number of successional areas from 1913 to 1916. Some of the general 



216 CONCEPT AND HISTORY 

results have already been published (Bergman and Stallard, 1916; Stallard, 
1916; Bergman, 1919; Stallard, 1919), while a part of the indicator findings 
are discussed later (Chapter XIII). 

Shantz, 1911. — The study of the natural vegetation of the Great Plains 
by Shantz is the classic work on indicator plants. It was the first avowed 
investigation of indicators to be based upon the three cardinal points, namely, 
instrumentation, succession, and quadrats, and will long serve as the model 
for all thorough research in this field. Because of its great importance, the 
original should be consulted for the details. Here it must suffice to quote 
the author's general principles, (p. 9.) 

"Farmers and other persons who have occasion to examine new land in 
order to form a judgment of its agricultural value depend largely upon the 
natural vegetation, or plant covering, as an indicator of its crop-producing 
qualities. But there are many possibilities of error in judging land upon this 
basis. Species that are closely related botanically and very similar in appear- 
ance may indicate quite different conditions of soil and climate. The popular 
names of plants are likely to cause confusion. Thus, the farmer who has 
learned in the Great Basin region that 'greasewood' is an indicator of alkali 
land and that 'sage-brush' usually grows on land free from alkali, will find if 
he moves to southern Arizona or southeastern California that the scrub there 
known as 'greasewood' indicates absence of alkali, while the so-called 'sage 
bushes' of that region grow on strongly alkali land. Furthermore, there is 
a general tendency to depend upon a single plant species as an indicator, 
while the investigations set forth in this bulletin show that the composition of 
the plant covering as a whole is a much more reliable basis for judging the 
crop-producing capabilities of land. 

"The chief object of the present paper is to show how these sources of error 
may be avoided and how new land may be classified readily and with reason- 
able accuracy on the basis of its natural vegetation. This paper is not a 
report of a land survey, but rather a discussion of methods which it is 
believed could be utilized to advantage in making such a survey, the methods 
being illustrated by application to a limited territory in the Great Plains area. 

"Too much emphasis can not be laid upon certain facts that have been 
clearly brought out in the course of these investigations: (1) Correlations 
between the natural plant cover and the crop-producing capabilities of land in 
a given area can be satisfactorily determined only after careful study of the 
different types of vegetation of the area in relation to their physical environ- 
ments; (2) such correlations, determined for some particular region, will need 
to be modified to a greater or less extent before they can be applied in another 
region where the physical conditions are different. When, as a result of suffi- 
cient investigation, correlations of this nature are determined for a given area, 
it is believed that they will afford a basis for classifying the land of that area 
more readily and at least as accurately as by any other known method. 

"In order to test and perfect the methods here described, it was necessary 
to make a detailed study of the vegetation of some particular area in relation 
to the physical conditions, checking the observations by the study of such 
examples of actual crop production as exist on the different types of land. It 
was decided to begin work in the Great Plains area, for this region contains 
the largest body of land in the United States having possible agricultural 
value on which the native plant covering is still undisturbed. A further 
advantage is the comparative uniformity of the climate throughout the area 
from the Canadian boundary on the north to the 'Panhandle' of Texas on 
the south. The investigations thus far have been made chiefly in a portion 



CLEMENTS 





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£:*$*«£ 



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A. Short-grass (lioiitcloita a rue ilia) on hard-land, Colorado Springs, Colorado. 
B. Wire-grass (Aristida purpurea) in short-grass subclimax, Walsenbnrg, Colorado. 



HISTORICAL 217 

of eastern Colorado, a region which is considered representative because of 
its central position and because its climatic conditions ;>re almost as a 
anywhere in the Great Plains. But enough data have been gathered in other 
portions of the Great Plains to make it fairly certain that with compara- 
tively little modification the correlations shown will hold throughout the area. 
"The work so far accomplished has brought out clearly that in this area 
the general conditions, whether favorable or unfavorable to crop production, 
are indicated by the character of the native plant cover." (plate 24.) 

Kearney, Briggs, Shantz, McLane, and Piemeisel, 1914. — The first quanti- 
tative study of plant communities as indicators of alkaline soils was made by 
Kearney and his a/aociates in the Tooele Valley of Utah. This was essen- 
tially an application of Shantz 's methods to a saline basin and met with 
similarly important results, as the following indicates: 

"In the arid portion of the United States the different types of native vege- 
tation are often very sharply delimited, the transitions being so abrupt that 
they can not be attributed to climatic factors ; this has suggested the possi- 
bility of correlating the distribution of the vegetation with the physical and 
chemical properties of the soil. If such correlations can be made, they may be 
utilized in the classification of land with respect to its agricultural capabilities. 

"One of the writers has described the correlations which exist in the Great 
Plains between the different types of vegetation and the physical characteris- 
tics of the corresponding types of land, and has pointed out how the native 
growth may be used in that region to determine the suitability of the land for 
dry-farming. 

"The results obtained in the Great Plains made it desirable to undertake 
similar investigations in the Great Basin region. The problems to be solved 
were: First, what types of vegetation indicate conditions of soil moisture 
favorable or unfavorable to dry farming, and second, what types indicate the 
presence or absence of alkali salts in quantities likely to injure cultivated 
crops. For the purpose of this investigation it was necessary to find a locality 
where both dry farming and irrigation farming are practiced, where much of 
the soil is still covered with the original native growth, and where some of 
the soils contain an excess of alkali salts. 

"After a reconnoissance trip through portions of Wyoming, Utah, Idaho, 
and Oregon in August, 1911, the Tooele Valley in central Utah was selected 
for the following reasons: (1) Several very distinct types of vegetation are 
found in a small area, (2) the soils show a great diversity in their moisture 
conditions and salt content, (3) the greater part of the area retains its origi- 
nal plant cover, while examples of crop production, both with and without 
irrigation, exist on different types of land. 

"Detailed studies of the vegetation of Tooele Valley in relation to the 
moisture conditions and salt content of the soil were carried on in 1912. The 
work was begun near the close of the rainy season (end of May) and was 
terminated during the first week of August, when the summer drought had 
reached its height. Additional data were obtained during a third visit to 
the valley in the latter part of August 1913. 

"The distribution of the native vegetation was found to depend in a 
marked degree upon the physical and chemical properties of the soils, factors 
which also influence crop production. So far as this particular area is con- 
cerned, the vegetation unquestionably can be used with advantage in classify- 
ing land with respect to its agricultural value. To what extent the correla- 
tions established in the Tooele Valley hold good in other parts of the Great 
Basin region remains to be determined by future investigation." (p. 365.) 



218 



CONCEPT AND HISTORY 



The successional relations of the dominants have been discussed as well as 
graphically illustrated by Shantz (1916:234). The primary succession ex- 
hibits two adseres, one from Salicomia and Allenrolfea to Artemisia, and the 
other from Allenrolfea through Distichlis and Sporobolus to Chrysothamnus. 
These serai facts give much additional value to the indicator studies of the 
Great Basin, especially in establishing the indicator sequence and in impart- 
ing a distinct significance to the various mixed communities. 



OUTSIDE (5) 

OUTER 8T1MULATE0 20NE (4) 
(WITH FRUITS) 



OUTSIDE (5) 
WITHERED ZONE <A a ) 



BARE ZONE (3) 




BARE ZONE (3) 



f$0&iy%) }/>;-:£ ''' vjxelTl^ '"-< 



I _ fc ^PLANT SURFACE 

S^fe* - " SOIL 8URFACE 



Fig. 10. — Zones of a fairy ring due to Agaricus tdbularis: A and C, during a moist 
period; B, during a dry period. After Shantz and Piemeisel. 

Shantz and Piemeisel, 1917. — In their exhaustive study of fairy rings in 
the Great Plains, Shantz and Piemeisel (1917:191) have shown the causal 
relation between the rings of mushrooms and grasses, as well as the indicator 
significance of the latter. They distinguish three types of fairy rings, based 
upon the effect shown by the vegetation: (1) those in which the vegetation is 
killed or badly damaged, caused by Agaricus tabularis (fig. 1) ; (2) those in 
which the vegetation is only stimulated, produced usually by species of 
Calvatia, Catastoma, Ly coper don, Marasmius, etc.; (3) those in which no 
effect can be noted in the native vegetation, due to species of Lepiota. In the 
Agaricus rings, the vegetation shows three zones concentric to the central 
area of normal short-grass sod (1) : the inner stimulated zone (2) is a broad 
one, differing in botanical composition, the more luxuriant growth, and the 
deeper green color from the center. The bare zone (3) is narrower and some- 
what more irregular, while the vegetation is either dead or consists of a few 
very poor perennials or short-lived annuals. The inner zone is the most 
prominent feature of the ring in spring or wet seasons, the bare one in late 
summer or fall or in dry seasons. The outer stimulated zone (4) is rather 
narrow and is made up for most part of species peculiar to the short-grass 
sod, though resembling the inner zone somewhat. The mushrooms occur in 
the outer zone near the outside edge. In the case of most fairy rings, the 



HISTORICAL 219 

fungus produces a temporary stimulating effect only, and the ring is indi- 
cated merely by the increased size, vigor, and chlorophyll-content of the 
annuals and the perennial grasses. 

The stimulation of the grasses and other plants which produced the inner 
and outer zones is probably due to the presence in the soil of nitrates and 
ammonia salts derived from (1) the reduction of the organic matter of the 
soil, (2) the decay of the mushrooms, and (3) the decay of (.he mycelium. The 
bare zone results from the death of the vegetation as a consequence of a lack 
of available soil moisture. Water penetrates very slowly into the sod filled 
with mycelium when it is once dry. The increased growth in the outer zone 
hastens the drying-out of the soil and, once dry, the latter is not wetted by 
heavy and continued rain. The vegetation is not noticeably damaged during 
growing seasons uniformly wet, but it quickly shows the effect of dry years or 
periods of drought. The secondary sere initiated by the fairy rings is essen- 
tially like that caused by any other disturbance in the short-grass association. 

Shantz and Aldous, 1917. — In the field instructions for classifying public 
lands under the terms of the Stock Raising Homestead Act of 1916, Shantz 
and Aldous have made the most comprehensive use of indicators for the pur- 
pose of land classification. Ninety different types are recognized as indicator 
communities and are described briefly, though usually without a statement of 
the correlated conditions. Of these, 32 belong to the prairie-plains grassland 
climax, 20 to the sagebrush climax, 16 to the desert-scrub climax, and 9 to 
the chaparral. The types are designated by the names of dominants and 
subdominants and represent both serai and climax communities. Density, 
percentage of grasses and grass-like plants, and height of shrubs are also made 
use of for minor indications, while overgrazed areas are given especial atten- 
tion. A key to correlation conditions and crop-producing capabilities was 
filed with the Geological Survey and is used by it in the interpretation of 
the types. 

Weaver, 1919.— While the work of Shantz (1911), Weaver (1915), Samp- 
son (1914, 1917), and of Cannon (1911, 1913, 1917), Markle (1917), and 
others had laid the basis for the consideration of root systems in connection 
with indicator values, the first special and comprehensive study of the indi- 
cator significance of roots was made by Weaver in 1918. This investigation 
derives its importance not only from the thoroughness of the methods, but 
especially also from the large number of species concerned, the wide range of 
the communities, and the consistency of the instrumental results. Approxi- 
mately 160 species were investigated, involving the examination of about 
1,150 individual plants. These were largely grasses and grassland herbs, but 
they included shrubs, undershrubs, weeds, and forest herbs as well. The 
communities represented were the prairies of Nebraska and the Palouse region 
of the Northwest, the short-grass plains and the sandhills subclimax of Colo- 
rado, the gravel-slide and half-gravel-slide associes, and the forest climax of 
the Pike's Peak region. In practically all these, readings were made of 
water-content, Immidity, temperature, and light, and in critical ones of 
transpiration as well. In showing the community relations of competing root 
systems use was made of the quadrat-bisect. Many of the detailed results 
have been utilized in the discussion of particular indicators in Chapters 
XIV and XV. 



220 CONCEPT AND HISTORY 

FOREST INDICATORS. 

A general idea of indicator plants has existed in forestry for nearly a cen- 
tury, and it is strange that the forester was not the first to formulate a system 
of indicators. His nearest approach to this is found in the tables of tolerance 
(Graves and Zon, 1911:20). The fact that the forester's attention was 
fixed primarily upon reproduction and little or not at all upon the shrubs 
and herbs of the forest floor probably explains the long absence of any definite 
recognition of indicators. In forestry as elsewhere, but even to a greater 
degree, a system of indicator plants and communities was impossible before 
the use of instruments and quadrats and the application of successional prin- 
ciples. As is shown later, however, forestry already possesses a large amount 
of indicator material which only needs to be organized upon a systematic 
basis. Practically all site studies have much and some of them great indicator 
value. However, the researches directed primarily toward this have been few, 
and it is necessary here to consider only the following: 

Cajander, 1909. — Cajander (1909; Zon, 1914:119) has made an interest- 
ing endeavor to recognize forest types on the basis of the living ground-cover 
as indicators of the soil conditions. He classified the forests of Finland, 
composed largely of spruce, fir, beech, and oak, into three types : 

1. Oxalis type (forests with a layer society of Oxalis acetosella). 

2. Myrtillus type (forests with a layer society of Myrtillus nigra). 

3. Calluna heath type (forests with a layer society of Calluna vulgaris). 

The Oxalis type characterizes the best soils and comprises nearly all the 
dominant trees. It is further divided into four subtypes, marked by Impa- 
tiens-Asperula, Asperula, Oxalis, and Oxalis-Myrtillus respectively. As Zon 
points out, the dominant species of trees are assumed to play no part in deter- 
mining the type. The author also dismisses the effect of light as of no impor- 
tance. This appears to be quite unwarranted, as no measurements seem to 
have been made of light, as is apparently true of the other factors as well, and 
consequently the correlation between communities and conditions is super- 
latively general. Little or no attention is paid to the successional sequence 
of dominants or subdominants, and here again the real indicator values are 
overlooked or lost. Zon further points out that the author's own statements 
are contradictory, in that he states in one place that the layer societies indicate 
the physical conditions independent of the tree species, while in another the 
trees are said to determine the character of the herbaceous vegetation beneath 
them. While Cajander has naturally assigned greater importance to the sub- 
dominant herbs and low shrubs than to the dominant trees, his use of the 
forest societies as indicators is sound, and will serve to correct the usual prac- 
tice of foresters who have neglected the undergowth. 

Clements, 1910. — The investigation of the lodgepole-burn forests of 
northern Colorado in 1907-1908 was essentially a study of fire indicators, 
herbaceous as well as woody. Its real importance in this connection lay in the 
fact that it was the first study of forests made on the complete basis of instru- 
ments, quadrats, and succession. It was pointed out that lodgepole pine and 
aspen are practically universal indicators of fire and not of mineral soil or 
other conditions, at least for the Rocky Mountains. Agrostis hiemalis, Chamae- 
nerium angustifolhim and Vaccinium oreophilum were recognized as the chief 



HISTORICAL 221 

pioneers of the burn subsere, together with the mosses Bryum argenteum and 
Funaria hygrometrica. Several other species are almost equally good indi- 
cators of burns, especially when abundant. These are Kubus strigosus, Carex 
rossii, Arnica cordifolia, Achillea lanulosa, and Anaphalis margaritacea. The 
water and light factors for the six dominant trees were measured and the 
successional sequence thus obtained exhibits the indicator value of each 
species. 

A successional study was made of the so-called natural parks of Colorado 
in 1910 for the purpose of determining their indicator significance as to refor- 
estation, both natural and artificial. The conclusion was reached that all 
such grassland areas in forested regions are but serai stages leading to a forest 
climax. The majority of them are due to repeated burns or the slow filling 
of lakes, with the result that they persist as apparent climaxes for several 
hundred years. Their origin is readily disclosed by the indicators in them, as 
is also true of the rate of development. 

Pearson, 1913-1914. — In discussing the proper basis for the classification 
of forest lands into types, Pearson (1913:79) has reached the following con- 
clusions : 

"The only scientific basis for such a classification is that of potential pro- 
ductiveness, considering both agricultural and forest crops. The productive 
value may be ascertained in two ways: The first measures directly, as far as 
possible, all physical factors on the site and gauges the productive capacity by 
the measure in which the sum of these factors meets the requirements of vari- 
ous crops. The second method uses characteristic forms of vegetation on the 
ground as an indicator of the physical conditions present, and upon this basis 
ascertains the adaptability of the site for different crops. The obvious objec- 
tion to the first method is the need of climatological data and soil analyses on 
each site to be classified; and owing to the diversity of sites in our forest 
regions, together with the almost complete absence of climatological records in 
many sections, the collection of the needed data would involve an expense 
which, at this stage of our advancement in forestry, would be almost pro- 
hibitive. The second method requires a thorough preliminary investigation 
in each region to be covered, in order to secure a working knowledge for the 
actual land classification, and obviously reliable results can only be obtained 
by the employment of trained men. This method is the simpler and probably 
the more reliable of the two, and it is considered entirely applicable to the 
needs of the forester." 

A general indicator relation is established between the five forest types and 
the agricultural possibilities of the Coconino National Forest in northern 
Arizona. The same author (1914:249) has employed seedlings of Douglas 
fir as indicators of the conditions for planting in aspen and in open situa- 
tions at 8,700 feet on the south slope of the San Francisco Mountains. The 
seedlings were planted in two plots in the aspen and two in the opening each 
spring of the 3-year period, and instrumental readings were made of water- 
content, evaporation, wind, and temperature. The aspen uniformly gave a 
larger survival of seedlings than the opening, the percentage varying from 
7 to 13. The critical factor in this was evaporation, which was 50 to 90 
per cent higher in the open than under the aspen. The author further 
points out that the results indicate that yellow pine, because of its lower 



222 CONCEPT AND HISTORY 

moisture requirements and greater demands for light, will probably prove 
more suitable than Douglas fir for openings within the natural range of the 
former. A later study has dealt with the correlation of height-growth with 
precipitation, but this is considered under growth-forms in Chapter XII. 

Zon, 1915. — At the suggestion of the writer, a conference was held at 
the Utah Forest Experiment Station in 1915 to discuss the feasibility of a 
system of indicators for silvics and grazing, and especially the indicator value 
of shrubby and herbaceous species and communities, with particular reference 
to succession. The conference consisted of Mr. Zon, chief of silvics, Mr. 
Jardine, inspector of grazing, Dr. Sampson, director of the station, Dr. E. S. 
Clements, and the writer. There was general agreement upon the value of 
indicators as a basis for the experimental regeneration of forest and grassland. 
As an outcome, Mr. Zon drew up a preliminary outline of the indicator sig- 
nificance of the important dominants of the various zones and represented 
this graphically in a schematic transect (fig. 25). This appears to have been 
the first definite organization of the indicator experience of the Forest Service 
in silvical work. Its proposals as to indicators are considered in Chapter XVI. 

A similar conference on indicators and succession was held at the station in 
1917. It was attended by Professor Tourney, Professor Pool, Dr. E. S. 
Clements, Dr. Sampson, Mr. Korstian, Mr. Baker, Mr. Weil, and other mem- 
bers of the staff, together with the writer. Particular attention was given to 
serai indicators of grazing burns, erosion and slides, as well as to climatic 
indicators in the chaparral belt. Some of the conclusions are to be found in 
the discussion of indicator papers in Chapter XVI, as well as in the body of 
the text itself. 

Hole and Singh, 1916. — In studying the reproduction of sal (Shorea 
robusta) in the forests of India, Hole and Singh have made a quantitative 
study of the water and light factors that control germination and ecesis. Their 
work is especially noteworthy in that experimental quadrats have been em- 
ployed for the analysis of different sites (p. 48), and that a detailed study 
was made of soil aeration as a critical factor. The general indicator results 
are given in the following excerpts : 

"Broadly speaking three principal soil types may be distinguished in these 
areas, and these are characterized by different types of vegetation, as follows : 

A. Containing a large percentage of sand and a relatively small percentage of the 

finer particles of silt. The soil is also frequently shallow, with gravel and 
boulders below, and is therefore essentially dry. 
Dry miscellaneous forest with Acacia catechu and Dalbergia sissoo prominent, or 
grassland with Saccharum munja dominant. 

B. Sal forest or grassland, well aerated deep loam with Saccharum narenga (often 

mixed with Anthistiria gigantea arundinacea) dominant. 

C. Badly aerated deep loam. This differs from (B) either in containing more clay 

and silt, in being actually denser with less pore space per cubic foot, or in 
having the water-table nearer the surface. 
Moist miscellaneous forest with Butea frondosa, Stereospermum suaveolens, Ter- 
tninalia, Cedrela toona and others, or grassland with Erianthus ravennae (often 
mixed with Anthistiria gigantea villosa) dominant. 

"One of these types is unsuitable for the growth of sal, inasmuch as the 
water-content of the soil falls rapidly to the death-limit after the close of the 
rainy season, while another type is unsuitable on account of bad soil-aeration 



HISTORICAL 223 

which leads to a low percentage of germination, a high percentage of deaths 
during the rains, and a superficial root system. The latter point is of great 
importance, inasmuch as it leads to the roots being situated in those layers of 
soil the water-content of which is reduced to the death-limit in the dry 
season. It will thus be seen that the results obtained go far to explain the 
natural distribution of sal, and also indicate those grasslands and forestless 
areas in which afforestation with sal offers the greatest chance of success. 
Finally, it has been shown that, owing chiefly to the heavy shade, the aeration 
of the superficial soil layers in dense sal forest is commonly below the death- 
limit for several weeks during the rains and that this factor is responsible (1) 
for the holocaust of sal seedlings which takes place during the rains in shady 
forests in years of heavy rainfall and (2) for the development of a superficial 
root system which, in the hot season when the sal sheds its leaves and the 
forest canopy thins out, leads to widespread damage from drought among 
those plants which survive the rains. Opening of the cover and temporary 
removal of the humus are obvious expedients by means of which the soil- 
aeration can be improved. Firing would also in some cases probably be bene- 
ficial in this respect." (p. 38.) 

' ' It will be seen that the management of any particular sal forest to a great 
extent depends on the fact whether the seedlings in it suffer chiefly from 
drought or from bad soil-aeration and therefore the determination of this 
point is of primary importance. Observations regarding the season when the 
seedlings chiefly die and the dryness of the soil at the time naturally indicate 
to a great extent which factor is primarily concerned. In addition to this, 
however, the work which has been carried out at Dehra during the last few 
years has shown that the dominant grasses on an area are, as a rule, excellent 
indicators of the soil conditions. Thus in northern India, where Saccharum 
narenga and Anthistiria gigantea arundinacea tend to be dominant, the soil 
moisture and aeration are as a rule suitable for the best development of sal, 
and sal forests of the moist type prevail. In shady forest in such localities, 
the seedlings suffer chiefly from bad soil-aeration and the most efficient remedy 
consists in opening the cover and exposing the soil. On the other hand, such 
grasses as Saccharum munja, 8. spontaneum, Eragrostis cynosuroides, Imper- 
ata arundinacea, Vetiveria zizanoides, Andropogon contortus, and Ischccmum 
angusUfolium usually indicate a soil too dry or too dense for the best sal 
development, and such forests as occur are of the dry sal type. The recogni- 
tion of the dominant grasses in the sal tracts therefore is a matter of consider- 
able practical importance, and a subsequent paper will deal in more detail with 
the grasses of the sal tracts, in their capacity as soil indicators." (p. 83.) 

Korstian, 1917. — In a study of permanent quadrats on the Datil National 
Forest of New Mexico, Korstian (1917:267) gives the increment data for 
Pinus ponderosa on sites I and II, and points out that the growth of a domi- 
nant tree is the best indication of the quality of forest sites. The differences 
in the native vegetation on the two sites were so great as to suggest its cor- 
relation with tree-growth and its use as an indicator of forest sites. A large 
number of list quadrats were employed, but the lack of previous successional 
studies makes their accurate interpretation difficult and probably explains in 
part the conclusion that 

"In studying the indicator significance of the native vegetation it is neces- 
sary to go directly to the individual species instead of attempting to stop at 
the association, society, or community. 

"The writer believes that the native vegetation found on deforested areas 



224 CONCEPT AND HISTORY 

may be considered as a criterion of the latent potentialities of the site for 
forest production provided the vegetation has not been too seriously or too 
recently disturbed and that the more important phases of the successional 
series are properly understood. 

' ' The fundamental study of forest planting sites logically resolves itself into 
three categories: (1) The empirical establishment of plantations and the 
observation and study of their survival and subsequent development; (2) the 
measurement and study of the most important physical factors of the site, 
such as the available soil moisture or growth water and evaporation; and (3) 
the indicator significance of the native vegetation occurring on the sites, im- 
plying a very careful correlation of all three phases. 

"It is readily conceivable that site studies of this character will be of the 
utmost value in explaining the presence or absence of tree growth on certain 
areas, in the judicious selection of the proper species and sites in the refor- 
estation of much of the denuded forest land of the United States, and in estab- 
lishing a working basis for the classification of forest lands. Only after con- 
sidering the relative agricultural and forest productivity of the land on a com- 
bined scientific and economic basis, can a positive conclusion be reached that 
its greatest utility lies in its use for forestry or for agricultural purposes. ' ' 

GRAZING INDICATORS. 

Grazing has been recognized as a distinct field for investigation for scarcely 
more than a decade. Complete recognition of grazing as a subject for experi- 
ment should perhaps be dated from the establishment of the Utah Forest 
Experiment Station for grazing in 1912. Three more or less marked steps in 
advance had preceded this and had made it inevitable. The first was a gen- 
eral study of the West with reference to the species, distribution, and value 
of the native grasses and forage plants. The stimulus for this seems to have 
been the work of Bessey in Nebraska, as indicated by the publication of many 
reports dealing with grasses and forage plants from 1886 to 1907. Webber 
(1890), Smith (1890), and Williams were associated with Bessey in some of 
this work and the last two later carried on extensive grassland studies over the 
Great Plains and the Rocky Mountain region (Smith, 1898; Williams, 1897, 
1898). Similar studies were made by Clements in 1893, Shear and Clements in 
1896, by Rydberg and Shear in 1897, by Pammel in 1897, Nelson in 1898, and 
others (cf. Shear, 1901). The second step was perhaps the most significant, inas- 
much as it introduced the quantitative study of grazing areas by means of the 
quadrat, and provided an exact method of measuring carrying capacity and 
determining the degree of overgrazing or the amount of regeneration. This 
work was begun by Griffiths and Thornber in 1901 and enlarged in 1903 on 
what is now the Santa Rita Grazing Reserve of the Forest Service. It has been 
carried on continuously since that time by Griffiths, Wooton, Thornber, Hurtt, 
and Hensel in turn, and this now constitutes the classic field for grazing study 
anywhere in the world. It has yielded publications of primary importance 
by Griffiths (1901, 1904, 1907, 1910), Thornber (1910), and Wooton (1916). 
Somewhat similar lines of experiment were begun by Coville and Sampson in 
1907 in the Wallowa National Forest in northeastern Oregon. The results 
are recorded in a series of reports of unusual significance, namely, Sampson 
(1908, 1909, 1913, 1917) and Jardine (1908). 

The third period of rapid development in grazing studies began with the 
organization of grazing reconnaissance in the six districts of the Forest Serv- 



HISTORICAL 225 

ice in 1911. During the past seven years reconnoissances have been made on 
practically all of the National Forests, and the grazing upon these has been 
administered upon the basis of a definite carrying capacity. The result has 
been to favor regeneration to such an extent that most of the ranges have 
recovered their normal carrying capacity to a large degree. With the exten- 
sive work in reconnaissance went the establishment of permanent quadrats, 
especially in the Coconino, Targhee and Deerlodge National Forests. Those 
on the Coconino especially have been actively studied (plate 42a), and have 
already yielded results of much value (Hill, 1917). 

The most signal advance has been marked by the organization of a grazing 
experiment station of the Forest Service at Ephraim, Utah, in 1912. This 
has been followed by the establishment of experimental pastures for grazing 
at Mandan (North Dakota), and Ardmore (South Dakota), by the Office of 
Dry Land Agriculture of the U. S. Department of Agriculture. Somewhat 
earlier than this, in 1908, Marsh had begun experimental work in Colorado on 
poisonous plants, and this is now carried on at a special experiment station 
at Salina, Utah, on the Fishlake National Forest. In 1914, the Jornada Graz- 
ing Reserve was established near Las Cruces, and this, like the Santa Rita 
Reserve, is essentially a grazing experiment station in the open range country. 
It seems inevitable that the organization of grazing reserves and experiment 
stations will proceed rapidly until they are found in all the important grazing 
types of the country, as well as in each State, including the South. An ac- 
count is given in Chapter XV of the inauguration of a comprehensive system of 
grazing investigations throughout the West during 1917-1919. 

Practically none of the grazing studies abstracted in the following pages 
was intended to deal with indicator plants. In spite of this fact, however, 
they all contribute more or less definitely to the understanding of grazing 
indicators, because of the simple and direct relation grassland dominants and 
subdominants have to grazing. In addition, the abstracts furnish a fairly 
complete outline of the progress of grazing investigations during the past 
twenty years. 

Smith, 1899. — The first clear recognition of grazing as a fundamental 
field for investigation was accorded by Smith in his study of grazing problems 
in the Southwest. His paper is a mine of valuable suggestions, and fore- 
shadows a large number of the later experiments. The author has a distinct 
idea of grazing indicators and of succession, as the following excerpts show : 

"Before the ranges were overgrazed the grasses of the red prairies were 
largely bluestems or sage grasses (Andropogon), often as high as a horse's 
back. After pasturing and subsequent to the trampling and hardening of the 
soil, the dog grasses or needle grasses (Aristida) took the whole country. After 
further overstocking and trampling, the needle grasses were driven out and 
the mesquite grasses (Hilaria and Bulbilis) became the most prominent 
species. The occurrence of any one of these as the dominant or most con- 
spicuous grass is to some extent an index of the state of the land and of what 
stage in overstocking and deterioration has been reached. 

"There is often a succession of dominant grasses in nature through natural 
causes, but never to so marked an extent as on the cattle ranges during the 
process of deterioration from overgrazing. Thus, the grasses in any given val- 
ley are liable to change in a long series of years through destruction by wood 

16 



226 CONCEPT AND HISTORY 

lice, prairie dogs, by fires, unusually early or late frosts, or by failure on the 
part of the plant to ripen seed. This later contingency frequently occurs in 
the case of the big bluestems and the feather sedge, and probably with some 
others of the Andropogon species. The curly mesquite will stand almost any 
amount of drought, trampling, and hard usage, but is easily killed and rotted 
out during a wet cold winter. The drought-resistant needle grass is frequently 
destroyed by wood lice over considerable areas. This usually happens in the 
spring on burned areas after light local showers. Fnally, the entire seed crop 
may be destroyed by early autumn fires. Thus it is seen that through some 
one of many natural causes a species of grass may be all but exterminated and 
its place taken by others, often of less value. 

' ' On overstocked land there is uniformly an alternation of needle grass and 
mesquite at short intervals, unless the overstocking is carried too far, when 
these perennials give way to annuals and worthless weeds. The carrying 
capacity then depends almost absolutely on the proper distribution of rainfall 
through the growing season in order to bring this transient vegetation to its 
fullest maturity." (p. 28.) 

The text is divided into the following heads: (1) investigation of carrying 
capacity, (2) destruction of grasses by animal pests, (3) deterioration through 
increase of weeds, (4) renewing the cattle ranges, (5) rest versus alternation 
of pastures, (6) additional aids to range improvement, (7) grazing regions in 
Texas and New Mexico, (8) relation of land laws to range improvement, and 
(9) benefits of improving the ranges. The most significant part of the report 
is that which has to do with the regeneration of the range by means of rota- 
tion pastures. Experimental sections were selected at Abilene and Channing, 
Texas, representing prairie and plains respectively. On these the following 
experimental pastures and areas were established (p. 20; Bentley, 1902:15). 

Pasture No. 1 (80 acres) : No treatment except to keep all stock off until June 1 

of each year, pasturing the balance of the season. 
Pasture No. 2 (80 acres) : To be cut with a disk harrow, and stock to be kept off 

until June 1 of each year, pasturing the balance of the season. 
Pastures Nos. 3 and 4 (40 acres each) : To be grazed alternately, the stock to be 

changed from one pasture to the other every two weeks, thus allowing the grasses 

a short period for recovery after each grazing. 
Pasture No. 5 (80 acres) : No treatment except pasturing until June 1 and keeping 

stock off the balance of the season. 
Pasture No. 6 (80 acres) : No treatment except to keep stock off during the first 

season. 
Pasture No. 7 (80 acres) : To be harrowed with an ordinary straight-toothed harrow 

and stock kept off during the first season. 
Pasture No. 8 (80 acres) : To be disked and stock kept off during the first season. 
Pasture No. 9 (70 acres) : Reserved for special experiments, viz., to determine 

(1) whether or not seeds of a number of wild and cultivated varieties of grasses 

and forage plants, exclusive of the grasses, could be sown directly in the sod with 

satisfactory results. (2) Whether the roots of certain sod and pasture grasses 

could be transplanted to the bare spots and a good stand secured in that way. 

(3) Whether the stand of grass could be improved by opening furrows across the 

pasture, in which the grass seeds blown over the ground by the winds could be 

arrested and the stand of grass be improved. 

Bentley, 1902. — The preceding experiments, though initiated by Smith, 
were carried out by Bentley from 1898 to 1901. His results are of great value 
as the first outcome of actual and successful experimentation in improving 
the range. At the beginning the maximum carrying capacity of the area was 



HISTORICAL 227 

determined to be 16 acres per head, or 1 : 16. During the first year, the 
carrying capacity was estimated to have increased to 1 : 8, or 100 per cent. 
Unfortunately, no detailed report was made on the different pastures, and it 
was impossible to tell whether rotation or disking and harrowing was of the 
greater value in securing these results. At the end of the second year, a fur- 
ther improvement of 30 to 50 per cent was noted in the disked pastures. By 
the close of the three-year period, while the whole area had improved more 
than 100 per cent, the greatest improvement was noted in the pastures which 
had been disked and harrowed. Two minor experiments of much practical 
interest were also carried out successfully. The one consisted of plowing 
furrows 12 feet apart over 10 acres of pasture 9. The many fruits caught in 
the furrows germinated readily and grew vigorously because of the increased 
water-content. The latter also benefited the grasses between the furrows. The 
other test involved the transplanting of grass mats and bunches for the pur- 
pose of covering bare areas in prairie-dog towns and other denuded areas. 
The results are of especial significance and are further discussed in Chap- 
ter XV. 

Griffiths, 1901, 1904, 1907, 1910, 1915.— Griffiths 's work upon the grazing 
ranges of southern Arizona from 1903 to 1910 is entitled to great credit as the 
earliest consistent study of range production. The quadrat method was em- 
ployed more or less, and some attention was paid to physical factors and 
incidentally to changes of population. The objects of the investigation were 
(1) to demonstrate that run-down and overstocked ranges will recover under 
proper treatment, (2) to ascertain how long a time is necessary to get appreci- 
able and complete recovery, and what methods of management will produce 
such results, (3) to carry on reseeding and introduction experiments in the 
hope of increasing the total quantity of feed, (4) to measure as accurately as 
possible the carrying capacity of a known representative area. The report of 
1915 on the native pasture grasses of the United States contains a large amount 
of valuable material with direct bearing upon grazing indicators. 

The general results of the investigations are shown by the following sum- 
mary (1910:24) : 

"The lands under consideration appear to regain their original productivity 
in approximately three years of complete protection. 

"Evidence thus far secured seems to indicate that the best lands in the 
vicinity will improve under stocking at the rate of one bovine animal to 20 
acres. The poorer lands take a correspondingly larger acreage for each ani- 
mal. The areas that will carry one head to 20 acres are very limited. 

"Brush and timber are encroaching upon the grasslands, due, it is believed, 
to protection from fires. 

"A ground cover is not a factor below an altitude of about 3,500 feet. 

"Although the maximum yield of forage may be reached in about three 
years of protection, improvements in quality of forage will probably go on 
longer through the continued supplanting of annual plants by perennials of 
greater value. 

"Thus far alfilerilla is the only introduced plant which has succeeded and 
this only in the most favored situations. It does not appear to thrive in com- 
petition with the native perennial grasses at those altitudes where the latter 
are not grazed. 

"None of the other 200 lots of seed sown has given any promise of success 



228 CONCEPT AND HISTORY 

except those of three or four native species. These give beneficial results, but 
the cost is high. 

' ' Results seem to be secured much more rapidly through proper protection 
from overgrazing than by any other method." 

Sampson, 1908, 1909, 1913, 1914. — The series of reports by Sampson on 
revegetation in the Wallowa National Forest constitute a contribution of the 
first importance to the science of grazing. They likewise furnish a large 
amount of experimental data as to grazing indicators in the montane and 
subalpine zones. The general results (1914:146) are applicable to a wide 
range of grasslands and are summarized below. They not only take into 
account the need of thoroughgoing and extensive studies of quadrats, factors, 
and succession, but they also consider in detail the ecological requirements of 
the various species. 

" ( 1 ) Normally the spring growth of forage plants begins in the Hudsonian 
zone about June 25. For each 1,000 feet decrease in elevation this period 
comes approximately 7 days earlier. 

"(2) In the Wallowa Mountains the flower stalks are produced approxi- 
mately between July 15 and August 10, while the seed matures between 
August 15 and September 1. 

" (3) Even under the most favorable conditions the viability of the seed on 
summer ranges is relatively low. 

" (4) Removal of the herbage year after year during the early part of the 
growing season weakens the plants, delays the resumption of growth, advances 
the time of maturity, and decreases the seed production and the fertility of 
the seed. 

"(5) Grazing after seed-maturity in no way interferes with flower-stalk 
production. As much fertile seed is produced as where the vegetation is pro- 
tected from grazing during the whole of the year. 

" (6) Germination of the seed and establishment of seedlings depend largely 
upon the thoroughness with which the seed is planted. In the case of practi- 
cally all perennial forage species, the soil must be stirred after the seed is 
dropped if there is to be permanent reproduction. 

"(7) Even after a fertile seed crop has been planted there is a relatively 
heavy loss of seedlings as a result of soil heaving. After the first season, 
however, the loss due to climatic conditions is negligible. 

" (8) When 3 years old, perennial plants usually produce flower-stalks and 
mature fertile seed. 

"(9) Under the practice of year-long or season-long grazing, both the 
growth of the plants and seed production are seriously interfered with. A 
range so used, when stocked to its full capacity, finally becomes denuded. 

" (10) Year-long protection of the range favors plant growth and seed pro- 
duction, but does not insure the planting of the seed. Moreover, it is imprac- 
ticable because of the entire loss of the forage crop and the fire danger result- 
ing from the accumulation of inflammable material. 

" (11) Deferred grazing insures the planting of the seed crop and the per- 
manent establishment of seedling plants without sacrificing the season's 
forage or establishing a fire hazard. 

"(12) Deferred grazing can be applied wherever the vegetation remains 
palatable after seed maturity and produces a seed crop, provided ample water 
facilities for stock exist or may be developed. 

" (13) The proportion of the ranges which should be set aside for deferred 
grazing is determined by the time of the year the seed matures. In the 



HISTORICAL 229 

Wallowa Mountains, one-fifth of the summer grazing season remains after the 
seed has ripened, and hence one-fifth of each range allotment may be grazed 
after that date. 

"(14) The distribution of water and the extent of overgrazing will chiefly 
determine the area upon which grazing should first be deferred. 

"(15) After the first area selected has been revegetated, it may be grazed 
at the usual time and another area set aside for deferred grazing. 

"This plan of rotation from one area to another should be continued, even 
after the entire range has been revegetated, in order to maintain the vigor of 
the forage plants and to allow the production of an occasional seed crop. ' ' 

Jardine, 1908, 1909, 1910, 1913. — Jardine has made a careful study of the 
relation of coyote-proof pastures to carrying capacity, and finds that the 
latter is nearly 100 per cent greater than under the usual method of herding 
in large bands. This is due to the fact that the sheep graze much more 
openly and do much less trailing, with the result that the vegetation is 
trampled very much less (1908:31, 1909:38). 

The establishment of grazing reconnoissances on the six forest districts and 
the organization of a method by Jardine in 1911 marked the beginning of an 
adequate system of grazing on the National Forests. This work has yielded 
a large number of facts of importance in connection with grazing indicators. 
Although it has never been published, its value is such as to warrant a brief 
abstract of it here. The main object of the reconnoissance was to secure a 
map classifying all the land of each National Forest into grazing types, and 
the location of each type, its carrying capacity and nature, whether winter, 
summer, or year-long range. The field notes dealt with the dominant species 
of each type, the density of ground cover expressed in tenths, the degree of 
utilization, and the presence of poisonous plants and range-destroying animals. 
Of most interest to the student of indicator plants is the system of types and 
subtypes which is outlined below. As quadrats gradually came into use in 
connection with reconnoissance, the later is now intensive to some degree in 
its methods. 



Typo 1. Open grassland other than 
meadow and secondary meadow. 
Subtypes: bunch-grass, grama grass. 
Type 2. Meadows. 

Subtypes: wet meadow, dry or sec- 
ondary meadow. 
Type 3. Weed. 
Type 4. Browse. 
Type 5. Sagebrush. 



Type 6. Timber, with a cover of 
grasses, weeds, and browse. 
Subtypes: pine-grass, weeds, browse. 
Type 7. Waste range. 

Subtypes : waste timber, waste 
brush. 
Type 8. Barren land. 
Type 9. Woodland. 
Type 10. Aspen. 



Wooton, 1915, 1916. — In his discussion of the factors affecting range man- 
agement in New Mexico, Wooton (1915:20, 23) has touched incidentally upon 
grazing indicators. The bulletin on the carrying capacity of ranges in south- 
ern Arizona (1916) continues the studies carried on by Griffiths from 1903 to 
1910. Five associations are recognized, and an interesting account is given 
of the secondary succession following plowing in the crowfoot-grama and the 
six-weeks grass communities. Of especial interest is the account of carrying 
capacity as determined by cut-quadrats, and by actual grazing tests in the 
various pastures. The conclusions are grouped tinder the following heads : 

Recovery. — The revegetation above 3,200 feet had become marked in about 



230 CONCEPT AND HISTORY 

three years after fencing. This improvement has continued, but more and 
more slowly each year, indicating that the normal condition is being reached. 
Below 3,200 feet, the rate of recovery has been slower and hence it should 
continue for a longer period. Three years of complete protection gave about 
three-fourths of complete recovery for the crowfoot-grama consociation with 
an annual rainfall of 15 to 18 inches. After 11 years the grazed areas are but 
partially recovered, though their carrying capacity has increased about 30 
per cent. 

Reseeding. — Practically all attempts to introduce new species of forage 
plants or to increase the abundance of endemic species beyond the normal 
have failed. Alfilaria and some aggressive annuals have given promise, but in 
the course of a few years the native perennials have crowded them out. 

Carrying capacity. — This has been determined by means of cut-quadrats, 
hay-cutting, mapping the communities, and by grazing tests of the best part of 
the reserve. For the latter, the carrying capacity is 14 acres per head, while 
it is 20 acres for the whole reserve. One of the pastures stocked on the basis 
of 58 acres per head was not noticeably different in condition from adjacent 
land protected for 11 years, thus indicating a utilization below 50 per cent. 

Jardine and Hurtt, 1917. — In the account of the results obtained on the 
Jornada Grazing Reserve from 1912 to 1917, Jardine and Hurtt have em- 
bodied the essentials of the first complete grazing system based upon actual 
experimental study of the herd as well as of the range. As a consequence, it 
serves as an excellent model for all ranches large enough to permit the rota- 
tion system of pastures and to warrant the segregation of herds by ages and 
classes. Taken in conjunction with the more intensive grazing experiments 
such as have been carried on by Sarvis (1919) at Mandan, it furnishes a com- 
plete experimental method of range studies. It is especially important in 
demonstrating how much experimental work and resulting improvement of 
range and herd can be carried on even under existing economic conditions on 
well-managed ranches (plate 36, a). 

The authors ' most important conclusions are as follows : 

The grama-grass range has improved at least 50 per cent in the three years, 
compared with adjoining unfenced range grazed yearlong. This has been 
secured by reducing the number of stock during the main growing seasons 
from July to October to about half the average number the area will carry for 
the year, by refraining from overstocking during the other eight months and 
by better distribution of watering places. The range thus lightly grazed dur- 
ing the growing season has apparently improved as much as similar range 
protected during the whole year. Where the whole of a range unit is grama, 
about one-third should be reserved in rotation for light grazing during the 
growing season for two successive years. 

Fairly efficient utilization of the range is secured by watering places with a 
2.5 mile grazing radius. When the distance is greater than this, serious over- 
grazing or actual denudation occurs around the well or tank, while the remote 
areas are but partially utilized. The carrying capacity of the grama grass is 
20 to 30 acres, of the tobosa grass 38 to 45 acres, and of the mountain range 
60 acres. This is based upon carrying stock through the average year in good 
condition, and feeding the poorer stock concentrates to eliminate loss from 
starvation at critical periods. 



HISTORICAL 231 

Jardine and Anderson, 1919. — In an account of range management on the 
National Forests, Jardine and Anderson (1919:17) have discussed briefly the 
general indicators of overgrazing: 

"Overgrazing for an extended period will leave 'earmarks,' which usually 
will be recognized. To recognize current overgrazing at the time of examina- 
tion on a range previously not overgrazed is difficult and yet important in 
order to make timely adjustment. The following obvious earmarks are the 
most reliable indicators of overgrazing prior to the year of examination : 

"The predominance of weeds and grasses such as knotweed (Polygonum 
spp.), tarweed (Madia spp.), mustard (Sophia incisa), annual brome grasses 
(Bromus hordeaceus, brizaeformis, tectorum) , and fescues (Festuca megalura, 
microstachys, confusa), with a dense stand of such species and lack of variety 
in species. This condition is a severe stage of overgrazing such as occurs 
around sheep bedding grounds which have been used for long periods each 
year for several years in succession. 

"The predominance of plants which have Utile or no value for any class of 
stock, such as sneezeweed (Dugaldia hoopesii), niggerhead (Rudbeckia occi- 
dentalis), yellowweed (Senecio eremophilus) , snakeweed (Gutierrezia saro- 
thrae) and gumweed (Grindelia squarrosa). These and similar plants fre- 
quently occur in abundance over large areas of range and indicate that the 
range needs careful management to give better forage plants a chance to grow. 

" The presence of dead and partly dead stumps of shrubs, such as snowberry 
(Symphoricarpos oreophilus), currant (Bibes spp.), willow (Salix spp.), serv- 
ice berry (Amelanchier spp.), birch-leaf mahogany (Cercocarpus montanus) , 
and Gambel oak (Quercus gambellii) . This condition usually indicates that 
the most palatable grasses and weeds have been overgrazed. There may be 
some exceptions to this, as in the case of dwarfed willows on ranges where 
grasses predominate above timber line. Sheep sometimes kill the willows 
before the grasses are overgrazed. 

"Noticeable damage to tree reproduction, especially to western yellow-pine 
(Pinus ponderosa) reproduction on sheep range and aspen (Popidus tremu- 
loides) reproduction on cattle range. Lack of aspen reproduction on a weed 
sheep range indicates overgrazing, provided the natural conditions are favor- 
able to aspen reproduction. On a sheep range where grass predominates 
severe injury to western yellow-pine or aspen reproduction may indicate that 
the range is not well suited to sheep. 

"The earmarks described are, perhaps, more typical of overgrazed sheep 
range than of overgrazed cattle range, but the general appearance of the two 
does not differ greatly when overgrazing reaches a stage to be recognized by 
one or more of these earmarks. The main differences are in the species of 
plants indicating the ovei'grazing. Weeds eaten by sheep are often found in 
abundance on overgrazed cattle range ; coarse grasses palatable to cattle are 
often abundant on overgrazed sheep range. This fact has given rise to the 
Use of the term ' class overgrazing. ' ' ' 

Sarvis, 1919. — The first adequate intensive experiments in grazing have 
been carried on by Sarvis (1919) at Mandan, North Dakota, since 1916, and 
at Ardmore, South Dakota, since 1918. These have dealt primarily with 
carrying capacity and rotation grazing, though a number of related problems 
have been taken into account, such as rate of growth, effect of mowing, etc. 
The experiments are based upon actual grazing tests to determine the present 
carrying capacity of a particular type and the optimum utilization resulting 
from rotation. At Mandan, for example, the carrying capacity tests comprise 



232 CONCEPT AND HISTORY 

four fields of 30, 50, 70 and 100 acres respectively, each grazed by 10 animals 
of the same age and class. These are weighed at frequent intervals and the 
carrying capacity expressed in terms of pounds gained in weight. There are 
three rotation pastures to permit grazing during one-third of the growing 
season — spring, summer, and fall respectively. The behavior of the commu- 
nity under the different degrees and kinds of grazing is measured by means 
of an unusually complete system of chart- and clip-quadrats. The details of 
the method are discussed in Chapter XV. 

CHRESARD AND WATER REQUIREMENT STUDIES. 

Significance. — While practically all studies of the chresard or available 

water in soils have been made without definite reference to indicator plants, 
it is clear that they have a direct bearing upon the latter. This is likewise 
true of researches upon water requirements, especially those that relate to 
controlling physical factors. Since the value of an indicator depends upon 
the exactness of its correlation with direct factors, and especially water, it is 
often totally misleading to relate it to obvious or superficial facts. For this 
reason a scientific system of indicators has but recently become possible. It 
was a distinct step in advance to connect species with the total water-content 
or holard. But this gives trustworthy results only for the same soil. To ob- 
tain exact results it has become necessary to determine the water-withholding 
power of different soils and the water-using capacity of different plants. It 
has likewise proved imperative to take into account the salt-content and air- 
content of the soil solution. In the further analysis of indicators, it proves 
desirable to utilize their form, growth, and abundance for more minute and 
exact values. Hence a knowledge of the growth requirements, which are 
largely water requirements, has come to be highly significant. 

Much work has been done upon the chresard of different soils and plants, 
and a still larger amount upon water requirements. Most of the former is 
American, and has been done in the "West. As a result, it has a direct bearing 
upon the problem under consideration here. Of the great mass of water 
requirement data only a few deal with native or non-cultivated species, and 
are pertinent to the present discussion. For these reasons a concise account 
is given of the progress of the chresard concept. 

The chresard. — The earliest studies of the water-content non-available to 
plants were incidental and failed to recognize the fundamental importance of 
the distinction. 

Sachs (1859, 1865:173) found that a young tobacco plant began to wilt in 
a mixture of sand and beech mold at 12.3 per cent and that the chresard for 
this soil was 33.7 per cent. A second plant in clay wilted at 8 per cent, with 
a chresard of 44.1 per cent, while for a third the echard in sand was 1.5 per 
cent and the chresard 19.3 per cent. Heinrich (1874) determined the echard 
of barley in peat as 47.7 per cent and of rye as 53.4 per cent. In calcareous 
soil corn wilted at 8.6 per cent and broad beans at 12.7 per cent. Mayer 
(1875) observed that pea plants wilted at 33.3 per cent in sawdust, 4.7 per 
cent in marl, and 1.3 per cent in sand, while Liebenberg found that beans 
wilted in loam at 10 per cent, in marl at 6.9 per cent, and in coarse sand at 
1.2 per cent. 



HISTORICAL 



233 



Gain, 1895. — Gain (1895:73) has studied the behavior of three mesophytes 
in six different soils, with the results indicated in the table below. The echard 
varies less than 50 per cent for these species in any one of the first three soils, 
but the variation rises as high as 60 to 130 per cent in the last three. Part of 
this may be due to a larger error in determining the low echard. The author 
concludes that species not only wilt at different points, but also that this varies 
for different stages of the development of the same species. 



Soils. 


Erigeron 
canadensis. 


Phaseolus vulgaris. 


Lupinus albus. 


Echard. 


Echard. 


Echard. 


I. 


II. 


I. 


II. 


I. 


II. 


Heath soil. 

Clay 

Humus . . . 
Lime soil. . 
Garden soil. 
Sand 


p. ct. 
9.26 
7.73 
6.80 
4.19 
2.30 
0.45 


p. ct. 
9.40 
7.78 
6.83 
4.25 
2.40 
0.48 


p. ct. 

10.73 
9.73 
6.10 
2.94 
1.79 
0.33 


p. ct. 

10.60 
9.58 
5.92 
2.90 
1.88 
0.35 


p. ct. 
10.90 
11.50 
6.86 
5.15 
2.82 
0.76 


p. ct. 
11.10 
11.35 
6.95 
5.23. 
2.91 
0.75 



Kihlmann (1890 : 105) was probably the first to perceive the ecological 
significance of the echard, in connection with his studies of water relations in 
the frozen bogs of Lapland. However, Schimper first recognized the universal 
application of the concept and formulated it definitely as follows (1898:3; 
1903:2): 

"It is necessary to distinguish between physical and physiological dryness 
and wetness; the physiological water-content alone is important for plant-life 
and hence for plant-geography. ' ' 

Neither Kihlmann nor Schimper appears to have made actual determina- 
tions of the physiological water-content. Clements (Pound and Clements, 
1900:167; Clements, 1904:23; 1905:30; 1907:13; 1916) developed methods 
for determining the echard and chresard in the field as well as under control. 
These were applied to various habitats in the prairie and woodland regions of 
Nebraska, and on Pike's Peak in Colorado. The general results were in accord 
with those of the earlier investigators, Sachs, Gain, and others, with respect to 
the variation of the echard with different species as well as with different soils. 
This led to a comprehensive investigation by Hedgcock (1902) of the echard 
and chresai'd of some 130 species under control, and 25 in the field. These were 
largely native and ruderal species, though a number of cultivated ones were 
included also. The great majority were mesophytes, though they ranged from 
xerOphytic grasses, such as Bouteloua gracilis, to such hydrophytes as Sagiita- 
ria and Potamogeton. The author reaches the general conclusion that "the 
ability of plants to take water from the soil varies in an ascending scale from 
hydrophytes through mesophytes to xerophytes. " 

Briggs and Shantz, 1912. — The most complete and thoroughgoing investi- 
gation of the echard has been made by Briggs and Shantz in connection with 
crop-plants for the Great Plains. Their methods and results are perhaps too 
well known to require comment, but it seems desirable to touch the latter 



234 CONCEPT AND HISTORY 

briefly for the sake of comparison. The term wilting coefficient is employed 
for non-available water or echard, but it is an exact synonym of these. The 
determinations of the echard for various soils are in essential accord with 
those of all other investigators, the values ranging from 1 per cent to 16 per 
cent, or in the heaviest clays to 30 per cent. But a striking departure from all 
previous results occurs with respect to the echard for different species. "While 
Heinrich, Gain, Clements, and Hedgcock found differences between species 
in the same soil represented by a ratio of 1 to 1.5 or 1 to 2, or even more in the 
case of hydrophytes, the greatest ratio found by Briggs and Shantz was 1 to 
1.1. The thoroughness of their work seems to leave little question of the 
soundness of the conclusion "that the differences exhibited by crop plants in 
their ability to reduce the moisture content of the soil before wilting occurs 
are so slight as to be without practical significance in the selection of crops for 
semi-arid regions." The issue must still be regarded as open with reference 
to material differences in the echard of native species, and this can only be 
settled by further research. Recent studies by Dosdall (1919) have shown 
that Equisetum differs greatly from Helianthus and Phaseolus in its ability 
to draw water from the soil, as was likewise demonstrated by growing them 
side by side in the same spots. In seeking to harmonize the discordant results 
of qualified investigators, it has become more and more probable that types of 
echard must be recognized. 

Water requirement. — In summing up the results of their own researches, 
as well as those obtained by many earlier observers, Briggs and Shantz (1913 : 
1:46; 2:88) reach the following conclusions: 

Experiments upon the effect of water-content on the water requirement 
show that the latter increases as a rule when the water-content approaches 
either extreme. 

A reduction in water requirement generally accompanies an increase in the 
nutrient-content, while a higher water requirement may result from a defi- 
ciency in the amount of a particular nutrient. 

The type of soil affects the water requirement only though the water or 
the solutes it contains. 

The water requirement increases with the dryness of the air, and is pro- 
foundly affected by climatic conditions. 

The water requirement varies greatly for different species and varieties. In 
Colorado, it was found to be approximately 1,000 for alfalfa, 700 for sweet 
clover, and 300 for millet and sorghum. The grains ranged from 369 for corn 
to 507 for wheat and 724 for rye. 

The greatest value of water requirement work for indicator studies is in 
connection with the phytometric analysis of climates and habitats. So far as 
the water relation is concerned, the values obtained by means of phytometers 
can be expressed in terms of water-loss per unit area or rate of growth, or in 
the water requirement in terms of dry weight or seed production. For crop 
plants, the latter are the most important, but for native species all four values 
must be taken into account, in addition to photosynthetic efficiency. 

CONCEPT. 

General. — Every plant is an indicator. This is an inevitable conclusion 
from the fact that each plant is the product of the conditions under which it 
grows, and is thereby a measure of these conditions. As a consequence, any 



CONCEPT 235 

response made by a plant furnishes a clue to the factors at work upon it. 
"While this general principle seems to be of universal significance, its applica- 
tion is far from simple. This is because the most direct responses are physio- 
logical and for the most part can be determined only by experiment. Such 
complex physiological processes as growth and reproduction are exceptions 
inasmuch as they are subject to direct observation. Consequently they are 
among the most valuable of indicator evidences. Structural responses are 
the most visible of all, but their exact use is the most difficult since they stand 
at the end of the process initiated by the causative factors. Structure also 
possesses a well-known inertia, as a result of which it may register the impact 
of factors but partially or slightly. Moreover, the adaptation to the habitat 
may be made in the tissues of the leaf without affecting the gross features to 
an appreciable degree. A plant may show the most exact response to chang- 
ing conditions by the behavior of chlorenchyma or stomata, and yet reveal 
no sign of this in its outward appearance (E. S. Clements, 1905). 

The interpretation of indicators is profoundly affected also by the double 
complex of factors and plants. The species of a community do not always 
register the same response, nor do they respond to any one factor in the same 
degree. The habitat itself is still largely a puzzle, and it is often difficult to 
assign well-marked effects to definite causes. The behavior of individuals, 
though manifestly of less importance, is not without its difficulties. It is 
impossible to tell at present whether the varying behavior of individuals of 
the same species is due to individuality or to minute differences in the habitat. 
Hence, the problem of indicator values is chiefly one of analyzing the factor- 
complex, the habitat, and of relating the functional and structural responses 
of both the plant and community to it. This then makes possible the accurate 
employment of indicators in practical operations. 

Animals as indicators. — Since their response is direct, plants are the best 
indicators of physical processes and factors. They are by no means unique 
in this respect. Animals likewise show direct responses to physical conditions 
and to this extent serve as indicators of them. For a number of reasons they 
are inferior to plants, however. The chief reason is that their significance is 
subordinate to that of plants because the latter as food-supply usually con- 
stitute the controlling factor. In other words, animals are as a rule indicators 
of plants more directly than of physical conditions. Their mobility makes 
the control of a particular habitat or set of conditions less absolute, especially 
with land animals. With the exception of insects, land animals are much less 
abundant than plants, and the indications of an animal community are much 
less complete and definite. Finally, our knowledge of the ecology of animals 
is much less than that of plants, especially with reference to factor control and 
succession. In spite of all this, however, animals do have great indicator 
value, second only to that of plants. While the time has not yet come for an 
adequate treatment of them in this connection, they are taken into account 
at various points in the text. Indeed, any other course would be illogical in 
view of the conviction that the complete response to habitat is the biome, or 
community of both plants and animals. 

Plant and community. — It has already been suggested that the individual, 
the species, and the community are all involved in the indicator concept. 



236 CONCEPT AND HISTORY 

Each of these has its own value, while all of them must be taken into account 
sooner or later. Up to the present, the species has almost monopolized the 
role, though the work of Shantz (1911) in particular has emphasized the im- 
portance of the community as an indicator. In constructing a complete scale 
of indicator values, the individual will play a necessary part. Its indications 
are more minute and subject to greater error. While further quantitative 
work will increase the accuracy and usefulness of individual indicators, at 
present they are distinctly secondary. In fact this will probably always be 
their relative position, inasmuch as they will serve to refine the major indica- 
tions of species and communities. The question of species and community 
values is much simpler than appears at first. It is not a matter of employing 
one to the exclusion of the other, but of taking advantage of their complemen- 
tary relation. There can be no doubt that the community is a more reliable 
indicator than any single species of it. This is a necessary consequence of 
the essential harmony of the important species as to physiological response 
and factor control. The community not only affords a better norm for the 
major indications, but it is likewise, so to speak, more finely graduated and 
hence more sensitive, owing to the fact that no two of its dominants or sub- 
dominants are exactly equivalent. It is also a better indicator of the whole 
habitat, since it levels the variations from one point to another. 

The indicator value of a species depends primarily upon its role in the com- 
munity. A secondary or subordinate species may be of little or no practical 
value, in spite of the general rule. It merely accompanies the major species, 
or as a subordinate accepts the conditions made by them, thus indicating 
minor differences. It assumes practical value only in case of the destruction 
of the dominants, as often happens in overgrazing and in deforestation. Even 
here the real meaning of a secondary species is due to the fact of its associa- 
tion with more important indicators. The significant species are the dominants 
and subdominants which give character to definite communities. With these 
the species and community values approach closely or merge completely. In 
fact such species give their typical indication only when dominant. Their 
incidental or scattered occurrences may have meaning, but it is not the normal 
one. In the present stage of our problem, then, attention should be focussed 
upon the dominants and subdominants of the climaxes and their various seres. 
When these have been correlated on the one hand with their efficient factors 
and on the other with practical processes in agriculture, grazing, and forestry, 
it will become evident whether an analysis of secondary species is profitable. 
The dominant may well be regarded as the real basis of indicator study, so 
commanding is its role in the processes of vegetation. 

Sequences. — Every indicator owes its value to its position in a cause-and- 
effect sequence. With this, however, must always be associated correspon- 
dence with another cause-and-effect sequence. The value of the compass- 
plant, Silphium laciniatum, as an indicator of corn production rests not 
merely upon its preference for relatively moist rich soils, but also upon an 
experiential knowledge at least of the production capacity of such soils. Up 
to the present, our knowledge of indicators rests chiefly upon the basis of ex- 
perience. In emphasizing the point that this alone is usually inaccurate and 
insufficient, there is no intention of failing to give it proper recognition. It is 



CONCEPT 237 

an essential and often the critical part of indicator research, but its true value 
can be obtained only by correlation with the other steps of the process. As a 
consequence, it makes little difference whether the approach has been through 
experience or investigation. Both must be taken into account before the 
exact meaning of any indicator is secured. For the future it is clear that 
much time will be saved by a method of investigation which replaces more or 
less vague experience by actual investigation. 

Direct and indirect sequences. — As is shown later, plants may indicate 
conditions, processes, or uses. The simplest of these is the first, the most prac- 
tical is the last. The plant may indicate a particular soil or climate, or some 
limiting or controlling factor in either. This would seem to be axiomatic, but 
it is well known that grassland, which is typically a climatic indicator, often 
occupies extensive areas in forest climates. Thus, the presence of a plant, 
even when dominant, is only suggestive of its meaning. It is necessary to 
correlate it with the existing factors and, better still, to check this correlation 
by experimental planting, or an actual tracing of the successional development. 

Indicators of processes usually require a double correlation, namely, that of 
the plant with the controlling factor, and that of the factor with the causal 
process, such as erosion, disturbance, fire, etc. Thus, in the Red Desert of 
Wyoming, roads through the sagebrush are marked by vigorous growths of 
Agropyrum. The latter is here a clear indicator of disturbance. From its 
usual position in adjacent lowlands, it is presumably an indicator of increased 
water-content as well. Actual instrumental study alone can determine the 
exact relation between the disturbance and the water-content, and between 
the water-content and the presence of Agropyrum. The indicator sequence 
is further complicated by the question whether the increased water-content is 
due to disturbance directly, to the elimination of competition, or to both. As 
a matter of fact, however, the field study of Agropyrum and Artemisia under 
a wide variety of conditions and in different successional relations indicates 
that disturbance acts through competition upon water-content. 

In the case of use or practice indicators, the sequence differs in accordance 
with the nature of the crop. When the crop is a natural one as in grazing, 
the sequence is simple and direct. This is especially true of grazing in which 
the value of the range is determined directly by actual experiential or experi- 
mental grazing tests, which establish the indicator value of each species. With 
overgrazing, the sequence is similar to that found in process indicators. 
Trampling disturbs the soil and destroys the less resistant plants. Both 
effects tend to increase the water-content of the soil and to give the advantage 
to such plants as Gutierrezia and Artemisia, frigida (Clements, 1897:968; 
Shantz, 1911:65). This relation is clearly recognizable in the field from the 
fact that Gutierrezia, for example, is characteristic of depressions, alluvial 
fans, roadways and other disturbed areas. In the case of forests, plants may 
seiwe directly as indicators of water or light values, or indirectly of disturb- 
ance such as lumbering or fire, and of such practices as reforestation and affor- 
estation. In these processes the crop is partly or wholly artificial, and the 
indicator sequence is essentially the same as for crop plants. This involves the 
correlation of indicator and crop plants with their respective habitats, and the 
close correspondence of the controlling factors in the latter. With forage and 



238 CONCEPT AND HISTORY 

grain crops, the sequence is more complex, partly because the species con- 
cerned are not native, but largely because the physical conditions are unnatural 
as well as controlled. As a consequence, while factor correlation and indicator 
correspondence are still important, the chief part must be taken by experiment 
and experience extending over a period of years. It is desirable if not essen- 
tial that this period be 12 to 15 years, in order to cover the range of condi- 
tions from the wet phase to the dry phase of a climatic cycle. This is 
particularly true in the use of indicators for land classification, in which graz- 
ing, forestation, and crop production must all be taken into account. 

Direction of indication. — The increasing attention paid to plants as indi- 
cators during the past decade has largely arisen from practical considerations. 
While this is highly desirable, it must be recognized that indicators have also 
a wide range of scientific application. Moreover, the more important and cer- 
tain practical values are made possible only through the ecological study of 
indicators. It is in the ecological sense that every plant is an indicator. The 
indicators of actual practice will be obtained by the selection of those which 
are the most distinctive and dependable. Thus, while the indicators for graz- 
ing, forestry, agriculture, and land classification will be established by more 
and more exact study, many indicators will find their chief use in ecology and 
related fields, which must lay the foundation for the scientific agriculture and 
forestry of the future. 

For these reasons, it is necessary to recognize that every dominant can be 
used as an indicator of past and future as well as of present conditions. This 
is due, of course, to the fact that every dominant or subdominant has a definite 
position in succession. As a consequence, it is an indicator not only of the 
plants which precede and follow it, but also of the soil conditions in which 
they grow. At the same time the definite existence of a climatic cycle makes 
it possible to relate growth and successional movements to climatic changes, 
both past and future, and to extend the application of indicators correspond- 
ingly. On the one hand, this enables us to greatly broaden and definitize the 
use of plants as indicators of soil, climate, and vegetational movements in the 
geological past; on the other, it permits us to look ahead and anticipate the 
changes due to climatic cycles and the development and movements of vegeta- 
tion and habitat. 

Scope. — A complete understanding of the broad significance of indicator 
studies must rest upon a recognition of the aims and methods of modern 
ecology. In the early characterization of this field (Clements, 1905:1) it 
was emphasized that ecology is the central and vital part of botany and that 
all the questions of botanical science lead sooner or later to the two ultimate 
facts, plant and habitat. These statements appear even truer to-day in the 
light of the progress made during the past twelve years. The one essential 
amplification is the inclusion of zoology, due to the growing conviction that 
the real unit of response to the habitat is the biological community. Further- 
more, it is desirable to place all possible emphasis upon the fact that ecology 
must fix its attention upon habitat and community in their natural relation. 
Finally, there must be the clearest recognition of the fact that the plant or 
animal must be the final arbiter in ecology, except of course in the vast field 
of human ecology. Fascinating and valuable as they are, instruments and 



CONCEPT 239 

quadrats are useful only in so far as they tell us what the plant, animal, or 
community is doing. The most complete records of climate, for example, 
have no merit in themselves. They acquire value only as the plant or animal 
discloses by its responses the factors or quantities which are effective or con- 
trolling. 

The threefold basis of ecology is factor, function, and form (Clements, 
1907 :1). As a consequence, every ecological fact has its indicator significance, 
and it becomes possible to determine these just as rapidly as factor correla- 
tions are made. The chief objective for the student of indicators is the cause- 
and-effect relation, and his chief task to show how effects may be used as signs 
of their causes. In a sense, the use of indicators reverses ecological procedure 
inasmuch as it leads from effects to causes. Sooner or later it involves a more 
or less complete system of reading all the evidence afforded by the responses 
of plants and animals, whether as individuals or communities. 

With respect to its application, the scope of indicator work is far-reaching. 
It not only furnishes a basic method in ecology, and especially in succession, 
but it is also equally applicable in paleo-ecology. Because it gives us the 
judgment of the plant upon the physical factors of the habitat, it is indis- 
pensable to studies of soil and climate in so far as they have to do with vege- 
tation. For the same reason, it is invaluable in land classification, and to the 
great plant industries, agriculture, grazing, and forestry. While this is truest 
of new regions, it holds to some degree for older agricultural communities as 
well. It applies with especial force to the great unoccupied or poorly utilized 
interiors of other continents, such as South America, Africa, Asia, and 
Australia, and is not without meaning for large stretches in Europe. In short, 
wherever plants grow, in field, forest, grassland, or desert, indicator results 
are always of some, and usually of paramount, importance. 

In their relations to succession and to climatic cycles, plants exhibit some 
of the most important indicator values. These involve quantitative relations 
of abundance and growth which in conjunction with factor determinations 
will give to ecology an accuracy and certainty more and more approaching 
those of the physical sciences. As a consequence, it will become increasingly 
possible to definitize ecological processes and principles, and to use them as a 
basis for accurately forecasting the behavior of plants under changed condi- 
tions. Such prophecy is possible at present in any region where an adequate 
study of succession has been made. Its scope will be extended and its proba- 
bility increased in just the proportion that instrumental, quadrat, and devel- 
opmental studies of vegetation become the rule. 

Materials. — As has been suggested earlier, while every study of the actual 
relation between habitat and plant is a possible source of indicator materials, 
only those are of real value which are based upon instrumental, quadrat, or 
successional investigations. The permanent foundation of indicator research 
must be laid by those studies which employ all three methods. For these 
reasons the published sources of indicator material are relatively few and 
recent. They are largely American and are confined almost wholly to the 
period since 1900. In fact, adequate ecological studies having indicator values 
as their avowed objective are all subsequent to 1910, and are largely due to the 
appearance of Shantz's paper on the indicator value of natural vegetation 
in 1911. As a consequence, the present treatise is of necessity based primarily 



240 CONCEPT AND HISTORY 

upon the investigations of the author during the past 20 years and of Shantz 
for the last decade or more. While these have had indicator plants as a 
definite objective only since 1908, the preceding 10 years of instrument, quad- 
rat, and succession work were an intrinsic part of the investigation. 

Basing studies. — Initial studies of grassland were made in Nebraska from 
1893 to 1898. These included a journey along the Missouri and Niobrara 
Rivers during the summer of 1893, one to the plains and foothills in 1897, and 
to the Black Hills in 1898. The first ecological expedition to Colorado was 
made in 1896, at which time a provisional outline of the plant communities 
was drawn up. Beginning with 1899, all the summers were devoted to inves- 
tigations in Colorado until 1913, with the exception of that of 1911, which was 
spent abroad. During the spring and fall from 1899 to 1907, studies in 
prairies and woodland in eastern Nebraska were carried on with the aid of 
advanced classes. The six summers from 1913 to 1918, inclusive, have been 
devoted to vegetation studies throughout the West, with especial emphasis 
upon succession, indicator plants, and climatic cycles. From 1912 to 1917, the 
work of the Botanical Survey of Minnesota was directed along similar lines. 

The use of quadrats was begun in 1897 and the instrumental analysis of 
habitats in 1898. The principles of succession were formulated into a working 
system for the field in 1898 (Clements, 1904:5), while studies of the echard 
and chresard were first made in 1900. The fundamental importance of the 
distinction between climax and serai communities was recognized in 1913, 
and the significance of climatic cycles in 1914. The two most recent advances 
that extend the use of indicators are the organization of the field of paleo- 
ecology in connection with the study of Badlands in 1915-16 and the formu- 
lation in 1916 of the concept of the biome as the basic biotic unit. 

Shantz (1906) began the ecological study of Colorado vegetation in 1903 on 
the basis of instrumental, quadrat, and successional methods. This led to the 
direct study of indicator plants on the Great Plains (1911) and in the Great 
Basin (1914). Out of this grew the extensive series of water requirement 
studies, as well as of transpiration, made by Briggs and Shantz between 1912 
and 1916. During the same period much attention was paid to western vege- 
tation, and this was crystallized in the list of indicator types for land-classifi- 
cation (Shantz and Aldous, 1917) and a map of the climax communities of 
the United States (Shantz and Zon, 1924). The text accompanying the map 
contains much information relating to the indicator value of the different 
vegetation types. 



XII. BASES AND CRITERIA. 

BASES AND METHODS OF DETERMINATION. 

Fundamental relations. — Plants serve as indicators by virtue of their 
response to conditions about them. Every plant response has some signifi- 
cance, the kind and degree of which must be subjects of exact determination 
in each case. Some responses are obvious, others less evident, while still others 
are invisible though demonstrable. All these, however, must be referred to 
the habitat for the decision as to their meaning and their possible use as indi- 
cators. It is clear that the causal relation of the habitat to the plant is the 
primary basis of plant indicators. Each response is the effect of some factor 
or factor-complex acting as a cause, and is consequently the indication of this 
factor. The chief task of the investigator is the measurement of responses, 
and their correlation with measured factors. 

In deciding upon possible bases for an indicator method, physiological 
responses and physical causes must be given the place of first importance. 
As further consequences of these must be considered the responses shown in 
the development and structure of communities, i. e., the basic facts of associa- 
tion and succession. The method of obtaining the facts in these four great 
fields will continue to be both empirical and experimental. Experiment will 
steadily increase in amount and value, but the result will be to refine and 
direct observation and not wholly to displace it. In fact, the more completely 
experiment is taken into the field, the more readily will observation reveal 
the meaning of the innumerable natural experiments brought about by changes 
of habitat and of climate. In this there is no intention of minimizing the 
crucial value of experimentation, but rather to widen its scope so that all 
experiments can be taken into account. This is especially important when 
one recalls the slow advance in experimentation under natural conditions and 
the insignificant area covered by it. The possibilities of this method have 
been strikingly shown for many years at the Alpine Laboratory, where numer- 
ous examples of natural transplanting in fragmented habitats verify and 
extend the results of a relatively small number of artificial transplantings. 
Similar results are to be obtained from natural experiments on a wider scale. 
The value of Bouteloua gracilis as an indicator of climate was graphically 
shown in the bad-land levels at Glendive, Montana, in 1917. The drying culms 
of the current year were just half as tall as those of 1916 which still persisted 
in the same mat. The rainfall for the two years was 26 and 12 inches, respec- 
tively. Thus the inevitable adjustment of the short-grass cover to decreased 
rainfall and water-content furnished results hardly to be surpassed by the 
most carefully checked experiment. 

In indicator work, as in all adequate investigation, by far the best method 
is that which uses all sources of information and does not emphasize one to 
the neglect of others. While the very nature of indicators insures proper 
consideration of habitat and plant, the study of each species must be accom- 
panied by that of its associational and successional relations, and all four of 
these objectives must be reached by the combined use of observation and 

241 
17 



242 BASES AND CRITERIA 

experiment, in which each must be utilized to the fullest capacity consistent 
with accurate results. 

THE PHYSICAL BASIS. 

Direct and indirect factors. — An adequate understanding of the habitat as 
the cause of plant responses that serve as indicators must rest upon two 
facts. The first of these is that the habitat is a complex, in which each factor 
acts upon other factors and is in turn acted upon by them. The second is 
that some of these factors are direct causes of plant response, while others 
can affect the plant only through them. Water, light, solutes, and soil-air 
are direct factors of the first importance because of their variation from habi- 
tat to habitat. Other direct factors, such as carbon dioxid, oxygen, and grav- 
ity, are negligible because of their constancy. Temperature is both direct and 
indirect, but its indirect action through the water relation is usually the most 
tangible. Wind, pressure, slope, exposure, soil texture, etc., are all indirect, 
acting for the most part through water-content or humidity, or through tem- 
perature upon these. 

Too much importance can not be given this distinction between direct and 
indirect factors. The indicator value of every plant depends upon it abso- 
lutely. A plant can only indicate a direct factor. But by the correlation of 
the latter with factors which are modifying it, the indicator response of the 
plant may be related to these. Thus, dwarfed herbs usually indicate a lack 
of water. In alpine regions this lack is largely caused by excessive transpira- 
tion and evaporation due to low pressure. As a consequence, dwarfs are 
typical indicators of high altitudes and hence of alpine climates. By other 
correlations of direct factors with causative processes, such as disturbance, 
erosion, cultivation, etc., plants come likewise to be used as process or prac- 
tice indicators. The true basis of all plant indicators is to be found in the 
responses made to direct factors, especially water, light, solutes, and soil- 
air. These once established, it becomes a simple matter to connect indicators 
with any correlated factor or process. 

Controlling and limiting factors. — It is evident that the factor in imme- 
diate control of the behavior of plant or community must be a direct one. 
But the latter may be profoundly affected by another factor in which the 
actual control may be said to reside. For example, montane timber-lines are 
often determined by water, but the availability of the water-content is decided 
by frost and its sufficiency by the wind. As indicated above, the immediate 
control and hence the immediate indication must be sought among the few 
direct factors, while the final control and indication will be found among the 
indirect factors that exert a critical effect. 

All the direct factors of the habitat play a part in the responses of the plant, 
but only those which vary widely in quantity leave a distinct impress upon it. 
This is necessarily true, since such constant factors as carbon dioxid, oxygen, 
and gravity produce fairly uniform responses, and consequently do not differ- 
entiate species or communities. In the case of each individual plant or species, 
its distinctive features are due to one of the variable direct factors. In prac- 
tically all cases at least one of these will be deficient, with the result that it 
becomes the limiting factor in the plant's development. This term is used in 
an ecological sense and not in the physiological one employed by Blackman 
(1905) and others. As a result the search for indicator correlations among 



THE PHYSICAL BASIS 243 

the four direct factors narrows itself to the one or two which are deficient. 
Some of these factors regularly bear an inverse relation to each other and all 
of them often show such a relation. Thus an abundance of water means a 
lack of oxygen, and a deficit of water a strong soil solution. Habitats deficient 
in light rarely show a lack of water or nutrients, though the oxygen-content 
of the soil may be low also. In practically all herbaceous communities, light 
is usually at the maximum, and the limiting factor must be sought in the soil. 
Hence, a careful scrutiny of many habitats narrows the search for limiting 
factors to a single one, and it is then possible to proceed at once with the 
quantitative correlation of factor and indicator. 

It must also be recognized that some factors limit plant response in conse- 
quence of an excess. This is true to some extent of solutes and water, but 
not of light or oxygen in nature. Even with the former, while the excess 
definitely limits or at least characterizes the plant's activity, the corresponding 
deficit of water in saline soils and of oxygen in wet ones or in ponds also plays 
a significant role. For water and solutes, it is probably more accurate to say 
that the extremes, either excess or deficiency, act as limits. While there are 
statements to the effect that full sunlight is directly injurious to many species, 
there is little or no conclusive evidence. This feeling has been based largely 
upon Bonnier 's work with alpine dwarfing, which has not been confirmed by 
similar studies in the Rocky Mountains. 

After eliminating the large groups of species that owe their indicator 
character to the limiting action of water, solutes, oxygen, or shade, there 
remains a much larger group of sun mesophytes which bear no such dis- 
tinctive impress. In a mesophytic habitat the four factors are present in a 
more or less balanced optimum. No one exists in marked deficiency or excess. 
Yet it has been demonstrated experimentally that a moderate increase in any 
one of the factors will be reflected in an increase of growth. Each factor in 
reality exerts a circumscribed limiting action as an outcome of competition 
between the plants. The various effects, however, are so moderate and so well- 
balanced that it is practically impossible to separate them. While water is 
usually paramount and light often the least important factor in the competi- 
tion between sun mesophytes, all four factors show a limiting action in at least 
a small degree. In spite of its apparent lack of a distinctive impress, a meso- 
phyte is as much the product of its habitat as the well-marked hydrophyte or 
halophyte, and serves equally well as an indicator. 

Climatic and edaphic factors. — The factors of climate and soil are so intri- 
cately interwoven in the habitat as to discourage analysis. For many reasons 
it is better to ignore such a distinction as of little or no significance to the 
plant and to fix the attention upon the cause-and-effect relation of one factor 
to another, quite independently of its location. This will reveal clearly two 
basic facts, namely, that the habitat is a unit and that the action of this unit 
is focussed upon plant and community by one or two limiting factors. The 
relation of the plant to water makes it evident that the distinction is merely 
one of classification which has no real significance to the plant. Water- 
content as a direct factor resident in the soil is directly or indirectly the result 
of precipitation, a climatic factor, and is profoundly affected by humidity, a 
climatic factor which it also influences. Its availability is determined by 
soil-texture, solutes, and oxygen, all soil factors, and by temperature, which 



244 BASES AND CRITERIA 

belongs to both soil and air, though in origin it is climatic. The baffling 
nature of the distinction has been well shown by Kaunkiaer (p. 6). In 
one sense, however, the distinction may possess some value. This is with 
reference to the factors which give character to the great areas marked by 
climaxes, in contrast to localized ones occupied by successional stages. It 
is more or less convenient to refer to such areas as climatic or edaphic, if 
it is recognized that the one denotes a permanent condition over a wide region 
and the other a relatively transitory stage in a restricted area. 

Moreover, the grouping of factors as physical and biotic appears to have 
little value beyond that of mere classification. Furthermore, it does not con- 
duce to clear thinking to use the same causal terms for the physical condi- 
tions which control plants and animals, and for the plants and animals them- 
selves. With the growing recognition of the community as consisting of both 
plants and animals, the true nature of biotic factors will become evident, and 
they will be recognized as reactions and coactions. 

Climates and habitats. — If one accepts the developmental basis for the 
study of vegetation, he must also admit the same process in habitats. Habitat 
and community develop reciprocally from extreme conditions to the fina] 
climax controlled by the climate. At this point climate and habitat become 
merged and are coextensive with the major community, the climax formation. 
In this connection, however, it is necessary to discard our ordinary ideas of 
climate and to accept the plant's view of what constitutes a climate. This 
fact has been appreciated by Wojeikov especially, in his work on the climate 
of beech (1910). The great grassland climax of North America lends particu- 
lar emphasis to the difference between climates as determined by plants and 
by man. In the human sense the climate of southern Saskatchewan is very 
different from that of northern Arizona, chiefly because of temperature, yet 
Bouteloua gracilis is an important grass in both places and the grassland 
formation is characteristic of both regions. Likewise the Palouse district of 
Washington and Idaho with its winter rainfall seems wholly different from 
the bunch-grass hills of Utah and the prairies of Nebraska; but if the vegeta- 
tion be taken as the indicator of climate, all three are essentially the same, 
since they are characterized by prairie associations (Weaver, 1914, 1917). 

The acceptance of the climax climate as the major or climax habitat enables 
us to establish a perfect correlation between habitat and vegetation. The 
climax habitat will show divisions corresponding to the association, and each 
association habitat may exhibit subdivisions in agreement with the consocia- 
tions. This is practically axiomatic, since each community is the product of 
the factor complex of its habitat. The habitat of one association must neces- 
sarily differ from that of another to the degree that one association does from 
the other. The subordinate communities of a formation, viz., societies and 
elans, also have their minor habitats, though these are less clearly marked, as 
would be expected. The structure of the climax climate or habitat corre- 
sponds closely if not exactly with that of the climax formation. It may be 
best illustrated by the grassland climax with its five associations, namely, the 
true prairie, mixed prairie, bunch-grass prairie, the short-grass plains, and 
desert plains. While all of these fall in the same climax climate, each one 
marks a corresponding division of it, or a subclimate. In the case of the true 
prairie, there are six dominants or consociations, Stipa spartea, Sporobolus 



THE PHYSICAL BASIS 245 

asper, 8. comata, Agropyrum glaucum, Koeleria cristata, and Andropogon 
scoparius, no two of them exactly equivalent as to habitat. Their require- 
ments approach each other so closely, however, that they occupy the same 
subclimate, in which they mix or separate in accordance with local variations. 
An interesting regional separation occurs with the two species of Stipa, as 
well as in the case of Agropyrum. Stipa spartea marks the eastern portions 
of the true prairies and S. comata the western; Agropyrum glaucum is typi- 
cally associated with Stipa comata, while A. spicatum is best developed in the 
Northwest, especially in the Palouse. The essential point is that each conso- 
ciation or mixture of two or more marks a subdivision of the association 
habitat, and is the indicator of it. Similar though minor habitat divisions are 
indicated by such characteristic societies as those of Glycyrhiza lepidota, 
Amorpha canescens, Psoralea argophylla, P. tenuiflora, Petalostemon candi- 
dus, and P. purpureus, the water relations of which are essentially in the 
order given here. In the eastern prairies, where water is abundant, several 
of these may occur together more or less constantly, but farther west each 
tends to form a distinct society, and to indicate a corresponding water-con- 
tent. The differences are slighter than in the case of consociations, and hence 
society habitats do not necessarily fall in the habitat of a particular consocia- 
tion. This is probably to be explained partly also by the action of climatic 
cycles. For example, the wet phase would favor the local extension of 
Psoralea argophylla and Petalostemon Candidas for a few years, while during 
the dry phase the less mesophytic Psoralea tenuiflora and Petalostemon pur- 
pureus would have the advantage. 

Since the habitat, like the formation, shows development in the course of 
succession, it exhibits developmental divisions and subdivisions. Each of 
these necessarily has its own indicator community, namely, the associes, con- 
socies, and socies. The habitats that correspond to these have a time as 
well as a space relation. If the best-known succession, the hydrosere, be 
taken as an example, these two relations are shown in the familiar zones of 
lakes and ponds. Each plant zone or associes from the center of submerged 
plants to the surrounding climax of forest or prairie indicates a major devel- 
opmental habitat, e. g., the habitat of the floating aquatics, of the reed-swamp, 
the sedge-swamp, etc. 1 Each of these associal habitats is subdivided into the 
habitats of consocies indicated in the reed-swamp, for example, by Scirpus, 
Typha, and Phragmites, respectively. Within the latter may be minor habi- 
tats characterized by such socies as Sagittaria, Alisma, Heleocharis, etc. As a 
result every region is a complex of climax and developmental habitats of vary- 
ing rank and extent, each controlling a plant community which serves as the 
indicator of it. 

Variation of climate and habitat. — While many reasons make it desirable 
if not necessary to regard each habitat as a unit, it should be clearly recog- 
nized that it varies from place to place and from year to year. The seasonal 

'Pearsall (1917:78) has recently recognized three associes of submersed plants, namely, 
(1) linear-leaved associes of Naias, etc.; (2) Potamopcton associes; (3) Nitella associes. 
This is in full accord with our growing knowledge of vegetational development, which 
must result in the general acceptance of more rather than fewer units (Clements, IPltf: 
132). However, the latter must be based upon quantitative studies and checked by 
extensive scrutiny of other vegetations if the results are not to be mere personal judg- 
ments, leading to the condition in which taxonomy finds itself today. 



246 BASES AND CRITERIA 

variations are more or less of the same character and they are marked by 
their own indicators in the form of the seasonal societies. A grassland climate 
is characteristically different from a forest climate by virtue of its product, 
the grassland climax. This has its explanation in the average difference 
between the controlling factors of the two during a term of years, but this 
difference is often less than that shown by the grassland climate in the dry 
and wet phases of the same climatic cycle. The rainfall of the wet phase if 
continued for a century or two under natural conditions would turn the 
prairie into forest, that of the driest period would under the same conditions 
convert it into desert. Similarly tbe distribution of rainfall is so erratic 
that two contiguous localities may show striking differences amounting to the 
success or failure of a particular crop. Progressive changes of rainfall, tem- 
perature, and evaporation occur with increasing altitude, latitude, and longi- 
tude. Further, each climate shades imperceptibly into the next, often through 
wide stretches. These are all elementary facts and the climatologist might 
well say that they are taken account of in the ordinary way of determining 
means or normals. As a matter of climatology this is true, but from the 
standpoint of indicator vegetation it is not. It is a simple matter to trace 
the line of 20 inches of rainfall, or of the 60 per cent ratio of rainfall to 
evaporation and to assume that it marks the line between prairies and plains. 
Such an assumption reverses the proper procedure, in which the associations 
themselves must be permitted to indicate their respective climates. When 
this has been done and the limits of the various communities established, it 
will be possible to determine the correlated factors. 

The real importance of climatic variations within a climax habitat lies in 
the fact that the correlations of vegetation and climate must be studied on 
the spot year by year. No single station can be typical of the whole habitat, 
and no year of the whole cycle. Yet for each station and for each year the 
indicator evidences of the vegetation should correspond closely if not exactly 
with the controlling factors. As a result, the study of representative locali- 
ties for each year throughout a climatic cycle should disclose the range of 
fluctuation in both climax habitat and vegetation, and establish all the indi- 
cator values of the latter upon a secure basis. 

The minute study of habitats reveals differences which are reflected in the 
behavior of plant and community, and hence cause the latter to serve as 
indicators. It is probable that every square foot of a habitat differs in some 
degree from every other one. Moreover, when the reactions of competing 
plants are taken into account, the differences are often more minute. In 
natural studies of competition made in Colorado and in California, as well as 
in competition cultures, differences of height and flowering have been found 
for each inch or two. Corresponding differences of density are of even more 
frequent occurrence in herbaceous communities. These indications have been 
checked by factor determinations only in a few cases as yet, but there can be 
little question that many more habitats show the most minute differences, 
each with the corresponding indication in terms of density, height, reproduc- 
tion, etc. In short, the indicator correlation of plants and habitats exempli- 
fies a universal principle which applies from the relation between climax 
formation and habitat through units of diminishing rank to the relation 
between the individual plant and its miniature habitat. 



CLEMENTS 




A. Lowland mosquito (Prosopis juliflora) at 2,500 feet in the San Pedro Valley, 

Arizona. 
B. Foothill mosquito meeting oak at 4,500 feet, Patagonia Mountains, Arizona. 



THE PHYSICAL BASIS 247 

Inversion of factors. — One of the early puzzles encountered in indicator 
studies, especially in connection with succession, was the occurrence of the 
same dominant in adjacent but diverse areas. This was first noted for Andro- 
pogon scoparius and Calamovilfa longifolia in sandhill and badland regions. 
These were found in rough areas and in blowouts on the one hand and in 
meadows on the other. While the serai relations were very different, the 
relation to water was much the same. On the broken or sandy ridges the soil 
was porous and the competition relatively small, due largely to the bunch 
habit, while in the moist meadows the grasses grew in a sod, the competition 
for water was keen, and the amount for each plant correspondingly limited. 
A similar inversion in hilly and mountainous regions has since been found for 
the majority of grass dominants, as well as for an increasing number of 
shrubs. The breaking-down of the Miocene rim of the Bad Lands of Nebraska 
and South Dakota yields a talus in which Rhus, Ribes, Symphoricarpus, Rosa, 
and other shrubs occur, all of which form dense thickets in the valley several 
hundred feet below. Chrysothamnus, Artemisia, and Atriplex grow far up 
the walls and buttes of bad lands, and are found again as dominants in the 
ravines and draws. In the Southwest the desert scrub consists of two major 
dominants, Prosopis and Larrea. While they are often mixed in the vast 
stretch over which they occur, Prosopis is typical of the valley and washes. 
The valley plains and bajadas are characterized by a zone of Larrea, above 
which lie Aristida-Bouteloua grasslands wherever broad sloping plains occur. 
In these Prosopis again occurs as a consequence of increasing rainfall, at an 
elevation of 1,000 to 2,000 feet above its position in the desert (plate 25). 

Similar inversions occur in mountain regions, either as a consequence of 
air-drainage or of exposure, or often indeed of both. In the case of exposure, 
the general relations are obvious, though the relative importance of water 
and temperature is usually uncertain. It seems probable that both are 
directly concerned, and that water plays the primary role, except in mountain 
regions characterized by a very short growing season and minimum night 
temperatures (of. Shantz, 1906^25; Shreve, 1915:64; Weaver, 1917:44). The 
effect of temperature inversions was pointed out by Kerner (1876:1) and 
Beck (1886:3) in Europe and has been studied by MacDougal (1900) and 
Shreve (1912:110; 1914:197; 1915:82). The latter 's conclusions are as 
follows (1914:115): 

"The influence of cold-air drainage might be expected to affect both the 
upward limitation of lowland species and the downward occurrence of mon- 
tane species. As a matter of fact the downward limitation of the forest and 
chaparral vegetation of the desert mountain ranges is due to the operation of 
the factors of soil and atmospheric aridity, and not to the chimenal factors. 
The limitation of the upward distribution of desert species appears to be 
attributable to chimenal factors, as the writer has shown for Camegiea gigan- 
tea. _ The writer has observed that a number of the most conspicuous desert 
species range to much higher altitudes on ridges and the higher slopes of 
canyons than they do in the bottoms and lower slopes of canyons. Samples 
indicate that there is no essential difference between the soil moisture of ridges 
and the bottoms of canyons during the driest portions of the year. Neither is 
there any evidence that desert species would fail to survive in the canyon bot- 
toms if they were somewhat higher in soil-moisture content. An explanation 
of the absence of desert species from canyon bottoms and their occurrence at 



248 BASES AND CRITERIA 

higher elevations on ridges must be sought in some operation of the chimenal 
factors rather than in the factors of soil and atmospheric moisture. An 
analysis of the operation of the chimenal factors will be sure to discover that 
cold-air drainage plays an important role in determining not only the lowness 
of the minimum, but also the still more important features of the duration of 
low temperature conditions." 

Measurement of habitats. — The importance of correlating indicator plant 
or community with the controlling factors of the habitat has already been 
emphasized. While the standard method of doing this has been by means 
of physical instruments, a number of attempts have been made to utilize 
plants themselves for this purpose. While the work of Bonnier (1890:514), 
in which he made reciprocal plantings of alpine and lowland plants, was essen- 
tially of this nature, he seems to have had no thought of using plants as 
instruments. The first conscious endeavor to do this was perhaps in 1906, 
when potometers of several different species were used with recording instru- 
ments to determine the effect of pressure on transpiration at different alti- 
tudes on Pike's Peak (Clements, 1907:287; 1916:439). Sampson and Allen 
(1909:45) employed sun and shade forms in different habitats at the Alpine 
Laboratory to determine transpiration in various light intensities, while 
standardized plants of Helianthus annuus were utilized in habitat measure- 
ments conducted by the Botanical Survey in Minnesota in 1909. During 
1912-1913, Pearson (1914:249) grew seedlings of Pseudotsuga beneath aspen 
and in openings to determine the better habitat for planting operations, and 
the method has since had a limited application by foresters. The most com- 
prehensive use of the planting method has been made by Hole and Singh 
(1916:48; cf. Chapter XIII), who established experimental quadrats in the 
sal forests of India to measure the role of shade and aeration in repro- 
duction. 

McLean (1917:129; cf. Livingston and McLean, 1916) employed soy beans 
to measure general climatic conditions by means of growth at two stations in 
Maryland. The three main criteria used in determining growth were leaf 
area, stem height, and dry weight of tops, all of which showed the Easton 
region to be nearly 2.5 times as efficient as the Oakland one. A definite corre- 
lation was established for temperature, but not for water, owing to auto- 
irrigation of the plants. Weaver and Thiel (1917:46) measured the trans- 
piration relation by means of bur-oak seedlings in three habitats, prairie, 
hazel-scrub, and oak forest, near Minneapolis. Similar measurements were 
made with maple and elm seedlings in scrub and prairie at Lincoln. Further 
experiments were made with sun and shade forms of the same species, and 
with sun and shade branches of the same plant. The species employed were 
Acer saccharinum, Ulmus americana, Fraxinus lanceolata, Rosa arkansana, 
Primus serotina, and Acer negundo. The general results showed a transpira- 
tion 2 to 3 times greater in prairie than in scrub and 6 to 10 times greater 
than in the Typha swamp. Evaporation was regularly greater than transpira- 
tion, and no constant relation was found between the two, as would be 
expected. Sampson (1919 :4) has recently made a comprehensive use of Pisum 
arvense, Triticum durum, and Bromus marginatus as standard plants in 
measuring the differences of the climax zones of the Wasatch Mountains in 
central Utah (cf. Chapter XVI). 



THE PHYSIOLOGICAL BASIS 249 

The use of plants to measure light intensities has as yet received almost no 
attention in spite of its great promise. This correlation has been made from 
the standpoint of adaptation by E. S. Clements (1908:83) ; when combined 
with growth and gross form, as in later studies, this method is simple and 
of great value. Even more significant is the use of standard plants for meas- 
uring light intensity and quality by means of the photosynthate produced 
in unit areas. Preliminary work of this nature has been carried out by 
Clements and Long (Clements, 1918:29; 1919; cf. Long, 1919) in the habitats 
at the Alpine Laboratory, and the chemical procedure has been refined to 
furnish a basic method of general application. The use of plants as instru- 
ments for habitat analysis is further discussed on a later page. 

THE PHYSIOLOGICAL BASIS. 

Kinds of response. — With rare exceptions a physical factor produces a 
functional response. Such responses are the most direct and the most accu- 
rate measures of the habitat, and hence would serve as nearly perfect indi- 
cators were it not for their being invisible. Fortunately, functional responses 
when marked regularly bring about structural changes which are visible. This 
is especially true of growth which, as the middleman between function and 
form, has the advantage of being direct as well as visible. Growth, like struc- 
ture, has the further merit of showing qualitative as well as quantitative 
differences and thus serves as an obvious record of abnormal response. From 
the standpoint of indicators, it is desirable to take all three kinds of response 
— function, growth, and structure — into account and to assign to each its 
proper value. The relative value is indicated by the sequence of the three 
as successive effects of controlling factors as causes. The rapidity and accu- 
racy of the response decreases with the distance from the impinging factors, 
while the readiness of its recognition correspondingly increases. As a con- 
sequence, indicator values have so far been based largely upon species and 
form. The importance of growth has later been recognized and it is but 
recently that function has been taken into account. In the further investiga- 
tion of plants as habitat measures and indicators, it is essential to determine 
the functional responses first, as the most direct and quantitative. These 
should then be correlated with growth measures and the latter with structural 
adaptations. "When this has once been done, either structure or growth can 
be used as ready and accurate measures, without resorting each time to the 
experimental analysis involved in functional measurements. As a matter of 
practical application, however, it is probable that growth and reproduction 
will serve as the best indicators of conditions for crop plants since the habitat 
is more or less controlled. In the case of forest and grassland, where the 
factors are essentially natural, a further analysis by means of functional 
determinations seeems desirable if not imperative. 

Effect of habit. — There are three reasons for the superiority of function 
over form for indicator correlations. The first is that considerable adjust- 
ments to factors can occur without affecting structure at all, the demands 
being fully met by functional responses. Another is that there is almost 
always a lag between function and structure, by which the effects of a factor 
appear in the latter only after a time or in diminished degree. These reasons 
are relatively unimportant compared with the role of habit, however, and the 



250 BASES AND CRITERIA 

second is perhaps only a consequence of the latter. While there has been little 
experimental study of habit as such, there are many suggestions of its impor- 
tance in modifying or reducing response, especially in structure. This influ- 
ence of habit is well known to foresters and agriculturists in connection with 
the germination of seeds from different regions and the behavior of their 
seedlings. It has also been shown in the case of alpine species transplanted 
to lower levels in that some retain the dwarf habit and others do not (Bonnier, 
1890), and for subalpine trees, some of which change their form and not their 
seasonal phenomena, while others reverse this behavior (Engler, 1912:3). 
The response of herbaceous species grown in two or more habitats is equally 
significant. Some are so responsive or plastic that both form and structure 
show practically perfect adjustment to each habitat in the first generation. 
Others modify the form and not the anatomy, and still others the interior 
of the leaf but not its form. There are all degrees of completeness of response 
to the stable plant, in which form and structure change little, and all the 
adjustment must be secured through function (E. S. Clements, 1905:93). 

As a consequence, the indicator value of any species can not be known until 
its functional response has been measured and correlated with the structural. 
This does not mean that the constant occurrence of a species in certain condi- 
tions can not be turned to practical account, but it does suggest the wisdom of 
regarding such correlation as tentative until the functional indication has 
been determined. The latter will also solve the puzzles presented by commu- 
nities in which very different life-forms, such as evergreen and deciduous 
trees, appear to flourish on equal terms. The most striking case of the mask- 
ing of the real response by habit is seen in such leafless rush-forms as Scirpus 
lacustris and Equisetum, in which it is now proved that the functional re- 
sponse is that of a hydrophyte (Sampson and Allen, 1909:49; Dosdall, 1919). 

Individuality in response. — Indicator values center about the species. 
Uniformity of behavior under uniform conditions and clear-cut adjustment 
when these are changed are the essentials of a good indicator. For these 
reasons it is important to deal chiefly with species which are represented by 
many individuals, such as dominants and subdominants, and hence to use the 
community as the basis for indicators. This makes it necessary to determine 
the range of individual response in function and growth as well as in struc- 
ture. In developing the use of standard plants as instruments, this matter 
is of the first importance. While the question of standardization will always 
enter, it will be convenient to use those species in which the individuality of 
functional response is slight. In the use of indicators, the range of individual 
behavior is a less important consideration than the knowledge of the range. 

Sampson and Allen (1909:37) have studied the individual behavior of four 
montane species as to transpiration and reached the following conclusion: 

"Only slight variations occur, not usually exceeding 3 mg. per square 
centimeter for a period of 12 hours. Therefore, it may be concluded that 
plants of the same species grown in the same habitat when tested under the 
same physical conditions show but slight variation in transpiration per unit 
of surface exposed." 

Effect of extreme conditions. — The significance of extreme conditions for 
response and the relation to indicator values is shown by the case of xero- 
phytes and halophytes. While the latter are now known to be merely 



THE PHYSIOLOGICAL BASIS 251 

xerophytes of a somewhat special type, they were long thought to constitute 
a distinct class. This is still true in a measure of those species which tolerate 
salts directly injurious, but it is well known that the majority owe their 
impress to physiological dryness due to the abundance of salts. But, while 
halophytes are indicators of arid conditions, it is a special type of aridity, 
and the indication must not be assumed to mean just what it does in ordi- 
nary soils. 

A somewhat similar case is afforded by the evergreen shurbs. In spite of 
the work of Kihlmann (1890:88, 105), it has been generally assumed that the 
evergreen shrubs of bogs, such as Chamaedaphne, Andromeda, Vaccinium, 
Ledum, etc., were xerophytes essentially similar in water relations to ever- 
green shrubs of arid climates. Recently the experiments of Gates (1914:445) 
have confirmed the conclusions of Kihlmann that while they are xerophytic, 
it is in response to physiological dryness in winter, and that they do not indi- 
cate aridity in such habitats during the summer. In fact, the summer indica- 
tions are rather those of deficient aeration. 

When growth is considered, the response of the same species to different 
extremes of one factor or another is often very similar. E. S. Clements 
(1905:93) has found in control experiments with Chamaenerium, Aquilegia, 
and Anemone that extremes of any factor which are not optimum for the 
species tend to dwarf plants growing in them. The general principle has 
been formulated as follows by Clements (1905:105): 

. "When a stimulus approaches either the maximum or minimum for the 
species concerned response becomes abnormal. The resulting modifications 
approach each other and in some respects at least become similar. Such 
effects are found chiefly in growth, but they occur to some degree in structure 
also. It is imperative that they be recognized in nature as well as in field and 
control experiment, since they directly affect the ratio between response and 
stimulus. ' ' 

This applies with especial force to the recognition of indicators, since their 
value depends primarily upon the close correspondence between response and 
the causative factor. 

Phytometers. — The best indicator of the nature of a habitat and of its 
practical utilization is the particular plant or community concerned. This is 
axiomatic, but it needs emphasis in connection with the experimental study of 
indicators. Such study may be made by means of physical instruments, 
standard plants, or the plants to be grown as a natural or artificial crop. The 
former is the simplest of the three, the latter the most effective. The use of 
standard plants combines the advantages of both to a large degree, and seems 
destined to undergo extensive development during the next few years. The 
refinement of method will lead to an increasingly wider range of possible 
standard plants, until it includes a large number of the species of greatest 
importance in agriculture, forestry, and grazing. Out of these will emerge a 
few species of broad powers of adjustment and adaptation which can be used 
as measures over great areas, such as between the associations of a climax 
formation or even between climax habitats themselves. A number of species 
of this sort are already clearly pointed out by their vast ranges and their 
vigorous growth in different regions. Of the grasses, Bouteloua gracilis, B. 
racemosa, Stipa comata, and Andropogon scoparius are perhaps the most 



252 BASES AND CRITERIA 

promising, and among shrubs Bhus trilobata, Cercocarpus parvifolius, 
Ceanothus velutinus, and Bubus strigosus. Of the trees, aspen is the best, 
with Pinus ponderosa and Pseudotsuga mucronata as the best of the conifers 
for the western half of the continent. As general standards, such weedy- 
herbs as Heiianthus annuus, Meliloius alba, and Brassica nigra are most 
useful. The most satisfactory cultivated plants are yet to be determined, 
but wheat, corn, and beans have obvious advantages. 

Preliminary results justify the feeling that standard plants or phytometers 
can be developed with more or less readiness to measure varying amounts of 
the direct factors, water, light, temperature, soil-air, and solutes. Such func- 
tional responses as transpiration and photosynthesis furnish the most accurate 
measurements, but growth responses are also of the greatest value, especially 
where factor-complexes are to be measured. Determinations based upon 
responses in form and structure are also distinctly valuable. Because of the 
longer time involved, they do not permit of such complete control, and their 
correlation is less exact. In all of these, the error due to individual behavior 
must be checked out by careful selection of individuals and by using a num- 
ber sufficiently large to yield a mode and to permit the elimination of those 
which depart widely. In addition it has proved increasingly desirable to use 
a battery of two or more species as phytometers, since this increases the num- 
ber and accuracy of the results quite out of proportion to the extra labor 
involved. 

The first application of the phytometer method was made by Clements and 
Weaver (Clements, 1918:288; 1919) at Pike's Peak in 1918 and 1919. The 
plants used were sunflower, beans, oats, wheat, sweet clover, and raspberry, 
Bubus strigosus. These were grown in sealed containers, with plants in open 
pots as checks on the conditions for favorable growth in the former. The 
normal number of pots for each species was 3 to 5, but this was often reduced 
by mishaps. Three series were grown during the summer, the period varying 
from 28 to 45 days. The habitats measured were those of the short-grass 
associes at 6,000 feet, the half -gravel associes, the gravel-slide associes, and 
the Pseudotsuga consociation at 8,500 feet, and the Picea engelmanni conso- 
ciation at 9,000 feet. Stations were visited each week for the purpose of 
making weighings and of reading the various recording instruments. The 
responses primarily considered were transpiration and growth, though photo- 
synthesis was measured also. These showed marked differences with refer- 
ence to altitude, degree of shade, and seasonal factors. The relative values 
were the same for the native Bubus as for the cultivated plants, and the 
complete results seem to leave no question of the paramount importance of 
plants for the quantitative study of habitats and communities (cf. Clements 
and Goldsmith, 1924). 

The use of several dominants in a phytometer battery amounts almost to 
employing a plant community as a measure, and suggests the possibility of 
utilizing portions of actual communities in this way. The simplest way of 
doing this at present is by means of permanent quadrats which are visited 
each month or each year and growth actually recorded by height or volume 
measures or by weight. Since many communities containing both dominants 
and subdominants, such as Stipa with Amorpha canescens, Psoralea tenui- 
flora, and Brauneria pallida, occur throughout the area of most climaxes, a 



BASES AND METHODS OF DETERMINATION 253 

series of quadrats containing essentially the same population can be estab- 
lished through a wide range of conditions. Locally, where diverse habitats 
are found within short distances, as in the case of zones about ponds and of 
dynamic areas, it is not difficult to transfer soil-blocks of the same com- 
munity to several different habitats and to follow their behavior in terms of 
the growth and abundance of the species concerned. Such communities afford 
the best possible measure of the serai habitats and reactions typical of suc- 
cession, especially when reciprocal transfers are made between two contigu- 
ous or successive stages (Clements and Weaver, 1924). 

THE ASSOCIATIONAL BASIS. 

Nature of association. — The association of two or more species in a com- 
munity is due to one or two of the following three reasons: (1) general simi- 
larity of functional response to controlling factors; (2) dependence upon the 
reactions of the dominants modifying these factors; (3) dependence upon 
the autophytes as hosts or matrices. The last two reasons also explain as a 
rule the presence of the animals of a community as well. Hence it is obvious 
why one species of a community should indicate the actual or probable pres- 
ence of the others regularly associated with it, and likewise the correspond- 
ing factors. This principle is susceptible of extended application, but it is 
nowhere more striking than in the case of relict herbs of a former forest. 
Though axiomatic, it must be used with some care, since no two species are 
exactly alike in response and indication, and since successional factors often 
enter to cause confusion. 

The occurrence of a dominant indicates not only the presence or possibility 
of its associated dominants, but also that of the related subdominants, secon- 
dary species, hysterophytes, and animals. This is as axiomatic as it is patent 
in the case of an actual community in the field. This relation becomes of real 
indicator significance where the community is partially or largely destroyed, 
when it is rapidly changing, or is but incompletely known, especially in the 
case of fossil vegetation. A subordinate species likewise indicates other sub- 
ordinate species as well as the controlling dominants, except in those plants 
which occur in two or more associations or formations, as well as in different 
serai stages. Even hysterophytes have a distinct indicator value when they 
are restricted to particular hosts. Moreover, it is clear that the associatioual 
relation signifies that animals may often be indicators of plants, as well as 
plants of animals. 

Dominants. — A dominant is the most important of all indicators. This is 
due to several reasons. The first of these is that it receives the full impact of 
the habitat, usually throughout the growing period. The second reason is 
that it reacts upon the controlling factors, and thus modifies the response of 
its associates. It also marks the progress of succession and consequently is 
bound up in a sequence of dominants, with the result that it affords both 
developmental as well as associatioual indications. In addition, it shows 
great abundance over extensive areas and occupies a wide range. In fact, 
its very dominance is the sign of its success under the conditions where it 
controls. However, it is necessary to recognize that a dominant species is 
not always dominant, and that its control may be local and developmental in 
parts of its range, while it is extensive and climax in the main portion. 



254 BASES AND CRITERIA 

Bouteloua gracilis is one of the most exclusive of climax dominants in its 
typical area, the short-grass community of the Great Plains, but it becomes 
a co-dominant or merely a successional one in the related associations of the 
grassland formation, and on the edge of adjacent climaxes, such as the 
chaparral and the sagebrush. In the Stipa-Sporobolus prairies it is subclimax 
on the ridges and drier slopes, while in the Aristida-Bouteloua desert plains 
it is usually subclimax also, but in the valley plains and swales it is truly 
climax. In all three associations it possesses indicator value as a dominant, 
but this value is different in each one, both as to its associates and the rela- 
tive conditions. Near the edge of its range it loses its dominance and becomes 
merely a subordinate member of the community with a greatly modified or 
restricted significance. 

The distinction between the dominance and the mere presence of a species is 
vital, from the standpoint of the structure of vegetation as well as from that 
of indicators. It is this which makes catalogues, lists of species, and general 
descriptions of the flora of a region of little value to the ecologist. In fact, 
such materials are trustworthy only in associations already known, where 
they are superseded. This is exemplified by a number of grass dominants. 
Bouteloua gracilis is found from Manitoba to Wisconsin and Mississippi, 
west to Texas, central Mexico, and California, and northward to Alberta and 
Saskatchewan. It occurs as the characteristic climax dominant of the short- 
grass community only in eastern Colorado, southwestern Nebraska, western 
Kansas and Oklahoma, northeastern Arizona, northern and eastern New 
Mexico, and in the Panhandle and Staked Plains of Texas. Usually with 
Bulbilis, it is more or less regularly associated with Stipa and Agropyrum 
from northwestern Nebraska and northern Wyoming through the Dakotas 
and Montana, into Saskatchewan. Altogether it is a climax dominant over 
perhaps a quarter of its range and a serai dominant over another quarter. 
Stipa comata is a climax dominant to-day only in Nebraska, northern Colo- 
rado, Wyoming, the Dakotas, Montana, and Saskatchewan, though it ranges 
from the latter to Nebraska, New Mexico, California, and northward to 
Alaska. As a consequence, the vegetational and indicator importance of any 
dominant species can be determined only by field studies of its abundance 
and role. Maps and conclusions based upon the distributional area alone are 
both misleading and erroneous. 

Equivalence of dominants. — The dominants of a formation owe their asso- 
ciation to the generally similar responses which they make to the climax habi- 
tat. This fact is further attested by the identity of life-forms and, to a small 
degree as yet, by actual measurement of the controlling factor. As the sum 
of similar responses, the formation is thus the largest and most distinctive of 
all indicator communities. Within the formation the dominants fall into asso- 
ciations by virtue of still closer similarity in response. Thus Stipa, Sporobolus, 
Agropyrum, and Koeleria constitute the climax prairies. By their height 
and general turf habit they indicate a rainfall of 20 to 30 inches. Bouteloua 
gracilis and Bulbilis dactyloides form the short-grass plains. Their short 
stature and mat habit are responsive to a smaller rainfall of 12 to 22 inches, 
which in effect is much reduced by evaporation. The Aristidas and Boute- 
louas of the desert plains from Arizona to western Texas are somewhat taller, 
but their bunch habit is an index of a smaller water efficiency, largely the 



THE ASSOCIATIONAL BASIS 255 

result of excessive evaporation. This relation is further indicated by the 
presence of Bouteloua gracilis in the moister valleys, and by the fact that 
Stipa and Agropyrum regularly mix with the short-grasses as indicated 
above, but have never yet been found mixed with the species of Aristida and 
Bouteloua characteristic of the desert plains. So far as our present knowl- 
edge goes, dominants of the same association or of the same associes are never 
exactly equivalent. Actually, they may seem to be since the annual varia- 
tions of the climatic cycle are often much greater than the difference in 
conditions. Even here, however, they tend to maintain their position or 
abundance, relative to the controlling factor. As a consequence, each con- 
sociation has its own indicator value, which, so far as its presence is con- 
cerned, necessarily varies somewhat from wet to dry phases of the cycle, but 
is checked by corresponding variations in growth, reproduction, and abun- 
dance. Thus, Stipa spartea and Agropyrum glaucum show climatic differ- 
ences from S. comata and A. spicatum, while Stipa comata and Agropyrum 
glaucum occur together over thousands of square miles, but are differentiated 
by water relations determined by soil and slope. The actual physical differ- 
ences in equivalence are slight, and hence the dominants of an association 
tend to mix or to alternate intimately instead of being pure over wide areas. 
However, this is necessarily truer of an association witli several to many 
dominants than of one with but a few (cf. Zon, 1914:124). 

Each dominant will grow in a fairly wide range of conditions, but will 
thrive only in a much narrower range. The field optimum for each is not a 
single point but an area. The areas of the dominants of the same association 
or associes overlap to such an extent that they coincide except at the ex- 
tremes. If the ranges of normal adjustment of Stipa comata and Agropyrum 
glaucum be represented in each case by a rectangle, the two rectangles will 
coincide for three-fourths of their lengths approximately. This indicates the 
degree of equivalence, the projections of each rectangle representing the 
actual difference in water-response for each species. This overlapping has its 
real counterpart in communities where the dominants are zoned. The mixed 
area between two zones represents the range of factors for which the two 
dominants are equivalent, and the pure zone on either side indicates the range 
peculiar to each. There is no necessary correspondence between the width of 
the zones and the mixed area, and the range of factor coincidence for the two 
dominants, owing to the varying rate at which such a factor as depth of 
water or amount of water-content may change. In the lakes of Nebraska, 
the two successive dominants, Scirpus and Typha, occupy the same depths 
from a few inches to several feet. Over most of this range they are mixed 
or alternating, but beyond 4 to 5 feet Typha drops out, while Scirpus may 
persist to a depth of 6 to 7 feet, Except where shores slope rapidly, the 
mixed zone is many times wider than the zone of pure Scirpus. 

In this connection it should be recognized that dominants show a wider 
margin between the normal range and better conditions than between it and 
worse conditions. In other words, a species is quickly and definitely limited 
by unfavorable factors, while those generally favorable to growth exert little 
limiting effect, the real effect being due to competition. This is the obvious 
explanation of the number of dominants and the abundance of species in 
sunnj'- well-watered habitats, such as prairies, open woods, alpine meadows. 



256 BASES AND CRITERIA 

etc., and their paucity in deserts and saline wastes. In short, abundance is 
itself an indicator, whether it concerns the individuals of one species or the 
species of a community. 

Absence of dominants. — The absence of a dominant from its particular 
community is often of indicator significance. A dominant may be lacking as 
a result of several different causes. Its absence may be due to unfavorable 
controlling factors, to very uniform conditions, to competition, destruction, 
or to the failure of invasion for any reason. In all of these cases except the 
last, absence has a definite indicator value, though it is practically always 
supplementary to the presence of its associates. This is perhaps its chief 
value, in that it enables us to check the positive indications obtained from 
presence. Absence due to unfavorable conditions or to competition is the rule. 
Uniformity of conditions, however, is a more frequent cause than has gen- 
erally been recognized. This is well illustrated by shallow lakes in the sand- 
hills of Nebraska, where the depth is so uniform that Scirpus is the sole 
dominant in spite of the fact that neighboring lakes show Typha, Zdzania, and 
Phragmites. Absence as a result of destruction is usually difficult to deter- 
mine and yet is of the greatest indicator importance. The grassy parks of 
the Uncompahgre Plateau in Colorado are so extensive and appear so perma- 
nent that their real significance, as well as that of the absence of the trees, 
was finally determined only by the discovery of burned wood deep in the 
soil. Similarly, much evidence has been found to show that the absence of 
Stipa or Agropyrum over wide stretches of the Great Plains reveals over- 
grazing of a type that has never been suspected. Thus, while absence is 
necessarily correlated with the presence of the related dominants in order to 
be usable, it does furnish indications of much value. 

Subdominants. — Subdominants are species which exert a minor control 
within the area controlled by one or more of the dominants of an association 
or associes. They are the successful competitors among the species which 
accept the conditions imposed by the dominants. As a rule they differ from 
the latter in life-form, and their competition is largely mutual rather than 
with the dominants. This is obviously the case in forests where the sub- 
dominants form layers. In grassland, where light controls in a minor degree 
alone, the layering is in the soil, but with a somewhat similar result that the 
dominants use the water before it reaches the deep-rooted herbs. In prairie 
and meadow, there is often enough water for both, a condition favored by the 
fact that subdominants reach their maximum at different times during the 
season, and hence cause the characteristic seasonal aspects. During dry 
phases of the climatic cycle, however, there is direct competition between 
dominants and subdominants, but usually at the expense of the latter. 

Within the limitations set by the dominants, subdominants follow the same 
general principles as to indicator values. This applies to their association in 
a community, either climax or serai, their equivalence, their dominance as 
compared with mere presence, and to their absence. They diverge, however, 
in exhibiting a seasonal sequence in many associations, by which they appear 
to escape too intense competition with each other. Prairies purple with 
Astragalus crassicarpus in April and May are covered with Amorpha, 
Psoralea, Petalostemon and Erigeron in June and July, and these in turn 
yield to golden rods, asters, and blazing stars in August and September. To 



CLEMENTS 





A. Pentstemon gracilis as ;i climax subdominant in mixed prairie, Gordon, Nebraska 

B. Pctlicularis crcintlota as a serai subdominant in a Jwicus-Carex swamp, Laramie, 

Wyoming. 



THE ASSOCIATIONAL BASIS 257 

a large extent these successive societies occupy the same ground and would 
seriously compete with each other were it not for the fact that the maximum 
demands of Astragalus, for example, are over before those of Psoralea and 
Erigeron begin. Societies thus have a time as well as a space value as indi- 
cators. While the subdominants of the same aspect are equivalent to a large 
degree, those of the three aspects, spring, summer, and autumn, differ in being 
progressively more xerophytic, owing to the seasonal relations of rainfall and 
evaporation. Societies are not only most numerous and best-developed during 
the early summer because of optimum conditions, but they likewise reach a 
maximum in those communities with optimum conditions, such as prairie and 
forest. In the short-grass plains they are greatly reduced, and in desert they 
are relatively few, except in the spring. This exception covers those deserts 
with two rainy seasons in which the socies of winter and summer annuals are 
possible only because of a relative excess of moisture near the surface at 
these times (plate 26). 

Secondary species. — This is here used as an inclusive term to comprise 
all the autonomous species of a community outside of dominants and sub- 
dominants. Their subordinate importance has caused them to receive rela- 
tively little attention, but their correlation with habitat factors has gone far 
enough to show that they all possess indicator value to some degree. In a 
sense, this is thrice removed from the habitat, since in climax communities in 
particular the conditions to which secondary species respond have been modi- 
fied by the dominants and then by the subdominants. Secondary species 
either make minor communities such as clans, e. g., Antennaria dioeca, Merio- 
lix serrulata, Anemone caroliniana, Delphinium carolinianum, etc., or they 
occur as scattered individuals in society or consociation. When they form 
more or less extensive clans which recur throughout an association, their 
indicator value approximates that of a subdominant. In fact, it must be 
recognized that some of the most important clans might well be regarded as 
societies. Or to put it more clearly, some subdominants vary sufficiently in 
abundance and control from place to place and year to year that they may 
form societies at one place or time, and clans at another. Apart from these, 
clans and scattered species have their chief importance in revealing minor 
differences of habitat within the consociation or society. They are often due 
to small disturbances and to succession in minute areas, and derive their 
indicator significance from this fact. It is probable that the careful study of 
secondary species will disclose some indicators of much sensitiveness and 
usefulness. 

Plant and animal association. — It is desirable for many reasons to con- 
sider animals an intrinsic part of the community as a biological unit. The 
great value of this is that it insures an adequate and correlated treatment of 
both plants and animals. It does not change in the least the basic relations 
between physical factors, plants, and animals, upon which their mutual indi- 
cator significance depends. Just as the plant indicates the factors and 
processes to which it responds, so does the animal serve as an indicator of the 
plant or community which furnishes it food, shelter, or building materials. 
The animal also indicates physical factors in so far as they affect it directly. 
The plant, however, has a double indicator relation by virtue of its response 
to factors on the one hand and of its control of animals on the other. Since 

18 



258 BASES AND CRITERIA 

animals are mobile for the most part, the control and the indications afforded 
by plants are necessarily less definite and exact. While the study of animal 
communities has gone far enough to provide a qualitative basis for plants and 
animals as reciprocal indicators, there has been no conscious endeavor to 
investigate this relation as yet. This is not true of paleontology, however, in 
which such causal relations as that between grassland and grazing animals 
have long been used. Even here an adequate and comprehensive system 
must await a fuller development of indicator values in present-day communi- 
ties. A preliminary attempt at such a system in both ecology and paleo- 
ecology is made in Chapter XIII. 

THE SUCCESSIONAL 'BASIS. 

Scope. — Since the nature of the habitat and the character of the popula- 
tion are constantly changing in all serai areas, succession is of profound im- 
portance in connection with indicators. While the basic rule that plants 
respond to the controlling factors holds for developmental as well as climax 
communities, the indicators change as the succession advances. Each stage of 
the succession is marked by factors that act upon species, which react in turn. 
Hence the indicator relations change more or less slowly but inevitably from 
one stage to the next. While the developmental areas of a formation are very 
much less in aggregate extent than those occupied by the climax stage, they 
are so numerous and various as to demand constant attention. The relative 
permanence of an indicator relation depends wholly upon whether it is deter- 
mined by developmental or climax conditions. Since the use of any area for 
cropping, forestation, or grazing either demands or effects constant changes 
in it, succession is the basis of all utilization of communities or dominants as 
indicators. This is especially true in the case of land classification, as Shantz 
has shown (1911:18), and it applies also to all engineering and construction 
operations in which the soil is disturbed or new habitats produced. 

Sequence of indicators. — Succession has been defined and analyzed as the 
development of a complex organism, the climax community or formation 
(Clements, 1905:199; 1916:3). It is a chain of causally related functions or 
processes. Development begins at certain definite points, pursues a regular 
course, and ends in the final or mature stage, the climax. As a result, each 
serai dominant or community has indicator values beyond those arising from 
the basic relation between plant and habitat. Each stage is the outcome of 
those that precede and the precursor of those that follow until the climax is 
reached. It indicates not merely the existing conditions, but it also points 
backward through successively remote stages to the beginning of the sere, and 
forward through those which lead up to the climax. Since the development of 
the habitat proceeds step by step with that of the formation, each stage is an 
indicator of earlier and later habitats as well as communities. Succession, 
moreover, is always progressive, and makes it possible to forecast not only the 
direction of development but something of the rate as well. It depends 
primarily upon the production of new, denuded, or disturbed habitats, and 
thus serves as an indicator of the many processes, physiographic, biotic, etc., 
which initiate new habitats or denude existing ones. 

The several indicator values of a serai community depend primarily upon 
the climax and the sere to which it belongs. The climax determines the domi- 



THE SUCCESSIONAL BASIS 259 

nants and subdominants from which the stages are drawn, indicates the 
climate in general control of the habitat changes, and constitutes the final 
stage toward which all the successions are moving. It is in itself an indi- 
cator of succession, since it permits the prediction of the general course of 
development that results from any disturbance in it. The division of seres 
into primary and secondary rests upon the double basis of habitat and devel- 
opment, and explains why each sere has indicator significance in itself. The 
primary sere or prisere indicates an extreme condition of origin, such as 
water or rock, slow reaction on the part of the earlier communities especially, 
and hence a large number of successive communities. The secondary sere or 
subsere begins on actual soil in which the conditions are not extreme, requires 
less reaction, exhibits few stages as a rule and runs its course to the climax 
with much rapidity. All seres, but primary ones in particular, are distin- 
guished upon the basis of the climax and the water relations of the initial 
area. The great majority of seres are mesotropic, that is, they progress to a 
mesophytic climax. In desert regions they are xerotropic and in the tropics 
may be hydrotropic ( Whitford, 1906) . Their indicator meaning varies accord- 
ingly, but it is even more subject to the water-content of the initial area. 
Seres are termed hydrarch (Cooper, 1912: 198) when they originate in water 
or wet areas, and xerarch when the initial condition is xerophytic or at least 
considerably drier than the climax. The nature and indicator value of 
hydroseres differ in accordance with their origin in lakes and swamps, or in 
bogs or other poorly aerated wet soils (oxyseres). Similarly, the indicator 
values of xeroseres vary with their origin upon rock, dune-sand, or in saline 
areas. 

Major successions as indicators. — The seres or unit successions discussed 
above are themselves parts or stages of greater successions. The cosere is a 
series of two or more unit successions in the same spot, and is best illustrated 
by those peat bogs in which the remains of the various stages and seres are 
accumulated in sequence and in position. In addition to the indications 
furnished by each sere, the cosere always indicates one or more striking changes 
of condition. "When it exists over a wide area or recurs in the same relation in 
several regions, it is an indicator of climatic change. An effective change of 
climate is denoted by the occurrence of the peat formed by water-plants as 
the layer above that which records the presence of the climax or subclimax 
trees. Such coseres have been industriously studied by European investigators, 
Steenstrup, Blytt, Sernander, Lewis, and others (Plant Succession, 378) and 
their climatic correlations established with much certainty. The record of a 
cosere is well preserved in water and especially in peat-bogs, but the more or 
less fragmentary records furnished by burns, dunes, moraines, and volcanic 
deposits are often of great value. This is especially true of the deposits of 
periods of great volcanic activity, such as the Miocene, as found in Yellowstone 
Park and the John Day Basin (Plant Succession, 367). 

Major changes of climate are accompanied by the shifting of climaxes as 
well as by the succession of seres in the same spot. The differentiation of 
climates during the Paleophytic and Mesophytic eras led to corresponding 
differentiation of vegetation with characteristic zones grouped around centers 
of deficiency or excess. These zones were clearly marked out by the opening 
of the Cenophytic era, since which time the major effects of climate have 



260 BASES AND CRITERIA 

been recorded in their shifting. It seems highly probable that the climatic 
cycles which produced and characterized the glacial period were accompanied 
by marked shifting of climax zones and that the close of the period left the 
primary zones of continents and mountains much as they are to-day. Such 
zones are the most striking and important of all climatic indicators, and their 
significance has been appreciated and investigated for more than a century. 
Perhaps even more important is the fact that such a series of shiftings or 
zones is a successional process by which it becomes possible to predict the 
general effect of any climatic cycle. This relation has already been developed 
to some extent , (Plant Succession, 347, 364) and is further discussed in con- 
nection with paleo-ecology (Chapter XIII). The greatest climatic changes of 
geological times are thought to be indicated by the evolution of the great 
land-floras and their differentiation into climax vegetations. Thus, the entire 
course of the development of the earth's vegetation, which is called the 
geosere, is divided into eoseres corresponding to the three great eras, and each 
eosere then exhibits clisere shifting in response to lesser cycles. The use of 
zones as indicator criteria is discussed in the next section. 

THE EXPERIMENTAL BASIS. 

Nature. — Indicators derive their importance chiefly from their practical 
applications. For all practical purposes, indicator values must finally be 
determined by experiment. The degree of their usefulness will depend mostly 
upon the kind and thoroughness of the experimental test. The planting of a 
trial crop by a settler will give some idea of the indicator meaning of the 
native vegetation that has been removed. In such a case the evidence is 
slight and its value tentative. If the planting is repeated for several years 
or is extended to other farms or localities, its value increases accordingly. 
As this is the usual course for a crop in a new region, it is obvious that 
ordinary agricultural practice must suggest indicator correlations with crop 
plants. This is well known to be the case, but the actual utilization of indi- 
cators by farmers seems always to have been inconsiderable. This is largely 
due to a lack of knowledge of native plants, especially in a new region, but 
also to the fact that this knowledge was needed most in selecting land and 
choosing crops, at a time when it was still to be acquired. Thus, while the 
aggregate experience of a neighborhood might possess real value, there has 
rarely been any method of formulating it and making it effective. 

The extension of experiment stations and substations throughout the West 
initiated the period of scientific study of agricultural problems. The investi- 
gations were directed chiefly to the selection of the best varieties for different 
regions and soils and to the improvement of yields. Unfortunately, the bota- 
nist was not interested in the problems of field crops and the agronomist was 
little or not at all concerned with native vegetation. The result was that a 
great mass of experimental data remained unavailable because it lacked corre- 
lation. It was possible to give this only through ecological studies, and then 
only after quantitative methods had been devised for the analysis of habitat 
and community. As a consequence, exact and purposeful studies on indica- 
tors date from the present decade for each of the three great fields, agricul- 
ture (Shantz, 1911), forestry (Clements, 1910), and grazing (Clements, 1916: 
102; 1917:303; 1918:296; 1919). In spite of this late beginning, the recog- 



INDICATOR CRITERIA 261 

nition and utilization of indicators are destined to undergo rapid development. 
This is especially true of forestry and grazing, owing to the fact that the 
corresponding experiment stations and reserves are organized upon the basis 
of exact ecology. 

Essentials. — It has already been insisted that experiment affords the only 
decisive test of an indicator. A single experiment may do this if properly 
checked, but repetition is regularly necessary to cover the range of conditions 
in space and in time. The experiment itself must be made with the fullest 
knowledge of the factors concerned as well as the vegetation to be correlated. 
As already pointed out, this involves quadrat study of the community and its 
successional relations, and instrumental study of the habitat and its variation 
through the climatic cycle. The thoroughgoing application of this method 
makes it possible to take advantage of countless natural happenings to convert 
them into experiments. The number of such possibilities furnished by denu- 
dation, lumbering, fire, cultivation, grazing, etc., is countless. If adequately 
utilized, they will not only greatly reduce the number of set experiments 
necessary, but will also make the latter possible on a scale otherwise out of the 
question. The natural experiment has the advantage in economy of time and 
effort, and in repetition of examples. The checked experiment permits of a 
definite choice as to time and place, and allows greater control. It is the essen- 
tial task of experimental ecology to combine these into a complete method, 
which will give quantitative results throughout the field of ecology as well as 
in connection with indicators. This is one of the primary objects of the pres- 
ent treatment, though the indicator relations are necessarily given first place. 

INDICATOR CRITERIA. 

Nature and kinds of criteria. — Every response of the plant or community 
furnishes criteria for its use as an indicator. These are most serviceable when 
they are visible, but demonstrable functional responses may be even more 
valuable, though invisible. The evidence as to functional responses in natural 
habitats is still very limited, and will be considered in the next chapter under 
the factors concerned. Here the discussion is confined chiefly to the criteria 
afforded by form and structure, with which growth is included. The develop- 
ment of the community is also considered along with its structure for the same 
obvious reasons. 

Criteria may first be divided into two kinds in accordance with their rela- 
tion to the individual plant or to the plant community. Individual criteria are 
phylogenetic when they have to do with species and genera, and ecological 
when they relate to life-forms and habitat-forms. It is probable that these 
are all ecological responses, and that species and genera are more remote in 
origin and hence their ecologic significance less evident. Life-forms are less. 
remote and their dependence upon the habitat more evident, while habitat- 
forms are mostly of more recent origin and their relation to the habitat obvi- 
ous. This view seems to be supported by the fact that it has proved impossible 
to make a system of life-forms which is not based in part upon taxonomic 
forms and in part upon habitat-forms. All of these criteria permit still finer 
analysis, as species into varieties and forms, and habitat-forms into those pro- 
duced by local or minute habitats. The experimental study of species and 
life-forms is still too slight for such a procedure, and it is possible as yet with 



262 BASES AND CRITEEIA 

only a small number of habitat-forms. The consideration of indicator criteria 
is based upon the following divisions: (1) species and genera; (2) life-forms; 
(3) habitat-forms; (4) growth-forms; (5) communities. 

Species and genera. — Quite apart from the life-forms and habitat-forms 
that they exhibit, species and genera, and to some extent families also, have 
an indicator value dependent upon their systematic position. The latter is 
determined primarily by the responses recorded in the reproductive structures 
at a time relatively remote. Their indicator meaning is consequently often 
obscure, and this obscurity is increased by a complete lack of experimental 
knowledge as to the factors which originate reproductive characters. Thus, 
while many species and genera show correlations with habitat or climate, this 
is chiefly on the side of vegetative responses, such as the relation of the Nym- 
phaeaceae to bodies of water. They often exhibit, however, a valuable indirect 
correlation with climate due to origin and migration. This is the basis of 
floristie studies such as those of Sendtner (1856), Drude (1890), and others, 
and of the more exact floristie methods of Jaccard (1901-1914) and Raunkiaer 
(1905-1916). The value of these must remain statistical and general until 
they are related to successional movements and to measured physical 
factors. 

Species and genera acquire their chief significance by virtue of the ecolog- 
ical values involved in phylogenetic relationship. This is obviously true of all 
genera which are largely or wholly consistent as to life-form, and it holds to a 
considerable degree for all others. Habitat, successional, and indicator values 
are concerned in this, and the genus thus becomes a sign of a more or less 
definite ecological complex of responses. This is likewise true of species in the 
general sense employed by Linne and Gray. A genus consists of several to 
many species because of the diverging evolution of an original stock under the 
more or less direct control of changing habitats. A species shows a similar 
evolution of forms, distinguishable from each other but mutually related to 
each other by descent, as are the species of a genus. For the ecologist, the re- 
lationship of such forms to the parent species is fully as important and even 
more significant than their recognition. It is imperative for his purposes that 
this relationship to the species be shown by the name as the latter shows that 
of species to the genus. This demands the use of trinomials, which is in accord 
with the general practice of ornithologists and mammalogists, but contrary to 
that of many systematic botanists. The one disadvantage of the trinomial is 
length, but this is readily obviated by using merely the initials of the specific 
name, e. g., Achillea m. lanulosa, Ranunculus f. reptans, Galium b. scias 
(Clements, 1908:263; Clements and Clements, 1913). This has long been the 
well-known practice of mammalogy and ornithology, e. g., Citellus t. parvus, 
Lepus c. melanotis, Cyanocitta s. frontalis, Butea o. calurus, etc. This or a 
similar method is inevitable if systematic biology is to aid and not hinder the 
development of ecology and the closely related practical sciences of agricul- 
ture, horticulture, forestry, plant pathology, economic zoology, etc. Three 
reasons would appear to lead irresistibly to this result. The field worker must 
deal with units which are recognizable in the field with a fair exercise of pa- 
tience and keenness. He must carry in mind the names and characteristics of 
a large number of species, and he can do this only by relating them to each 
other. There is a very definite limit to the capacity of the average memory, 



CRITERIA 263 

and this limit is greatly overstepped by a system which trebles the total num- 
ber of species in a region and substitutes for a clearly marked genus like 
Astragalus 19 genera recognizable with difficulty by the systematist and prac- 
tically impossible for others. Finally, while the ecologist is willing to go even 
farther than the systematist in recognizing minor differences, providing these 
are based upon statistical field studies and experiment and not upon herbari- 
um specimens, the practical scientist is concerned primarily with real species 
rather than the many varieties and forms into which some of them fall. At 
least, when the need for a closer knowledge arises in a particular case, it is 
infinitely easier and more helpful to deal with the variations of a well-recog- 
nized species than with a dozen binomials, none of which to him have the 
slightest relation to each other. 

If taxonomy is to be helpful to anyone but taxonomers, it must clearly do 
several things. It must recognize the field as the only adequate place for 
determining new forms, and must commit itself unreservedly to the methods 
of statistical and experimental study. It must restrict the use of the binomial 
to species in the Linnean and Grayian sense and employ the abbreviated 
trinomial for all segregates of such species, except in the rare cases where a 
coordinate species has been overlooked. It must realize that the splitting of 
genera only places so many stumbling-blocks in the way of all non-systematists, 
and makes them still more unsympathetic with such methods. Finally, it must 
recognize that a manual which can be used with success only by the syste- 
matist fails signally in its purpose, and be willing to construct keys and de- 
scriptions primarily for foresters, agronomists, grazing ecologists, and others 
whose knowledge of taxonomy is slight. Upon such a basis, species and genera 
will not only have vastly greater usefulness, but greater significance also to the 
ecologist, and he will be encouraged to do his share by handling them with 
greater accuracy and certainty (Hall and Clements, 1923). 

LIFE-FORMS. 

History. — The concept of the life-form was first formulated by Humboldt 
(1805:218), who used the term vegetation-form. Under various names, the 
concept has since been employed by many plant geographers and ecologists 
and several have proposed more or less complete systems of classification. 
Grisebach (1872), like Humboldt, based vegetation-forms upon physiognomy, 
and both systems have in consequence little more than historical value to-day. 
Warming (1884) and Reiter (1885) contributed many of the essentials of 
the modern systems, but these probably owe more to Drude (1890, 1896) 
than to anyone else. Krause proposed a classification in 1890 and Pound 
and Clements (1898) modified that of Drude somewhat in applying it to 
American vegetation. For this reason it is proposed to treat the latter here 
in detail, as well as the more recent systems of Raunkiaer (1903-1907), 
Warming (1908-1909) and Drude (1913). It will readily be seen that all of 
these have much in common, though this is not obvious in Raunkiaer's 
classification, which is based mainly upon adaptation for overwintering. All 
of them are founded more or less upon the two principles enunciated by 
Drude, namely, (1) the role played by a particular species in vegetation and 
(2) its life-history under the conditions prevailing in its habitat, with especial 
reference to duration, protection, and propagation. In the following discus- 



264 



BASES AND CRITERIA 



sion life-form is used as the general term to include vegetation- forms, habitat- 
forms, growth-forms, etc. 

Pound and Clements, 1898-1900. — As indicated above, the system employed 
by Pound and Clements in the " Phytogeography of Nebraska" (1898:45; 
1900:95; cf. Clements, 1902:616) was essentially the earlier system of Drude 
(1896) modified to fit the vegetation of a prairie State. It possessed some 
intrinsic interest in that the entire flora of the State was passed in review 
from the standpoint of the various groups, and with reference to the general 
conditions of the different habitats (1900:95-312). Vegetation-forms were 
arranged in 7 main groups, which were divided into 34 minor ones. This sys- 
tem was used by Clements and Clements in "Herbaria Formationum Colora- 
densium" in 1902 and "Cryptogamae Formationum Coloradensium" in 1906. 



I. Woody plants. 

1. Trees. 

2. Shrubs. 

3. Undershrubs. 

4. Climbers and twiners. 
II. Half shrubs. 

5. Half shrubs. 

III. Pleiocyclic herbs (perennials). 

6. Rosettes. 

7. Mats. 

8. Succulents. 

9. Creepers and climbers. 
Turf-builders. 

10. Sod-formers. 

11. Bunch-grasses. 
BMzomata. 

12. Rootstock plants. 

13. Bulb and tuber plants. 

14. Ferns. 

IV. Hapaxanthous herbs. 

15. Dicyclic herbs (biennials). 

16. Monocyclic herbs (annuals). 



V. Water plants. 

17. Floating plants. 

18. Submerged plants. 

19. Amphibious plants. 
VI. Hysterophytes. 

20. Saprophytes. 

21. Parasites. 
VII. Thallophytes. 

22. Mosses. 

23. Liverworts. 

24. Foliaceous lichens. 

25. Fruticulose lichens. 

26. Crustaceous lichens. 
Fungi. 

27. Geophilous fungi. 

28. Xylophilous fungi. 

29. Biophilous fungi. 

30. Sathrophilous fungi. 

31. Hydrophilous fungi. 

32. Entomophilous fungi. 
Algae. 

33. Filamentous algae. 

34. Coenobioid algae. 



Raunkiaer, 1905. — The system of Raunkiaer (1905 :347) seems on the 
surface to differ radically from all others. This is due to the fact that the 
winter protection of buds is assigned the first rank and the growth-form 
during the vegetative season is regarded as secondary. The apparent differ- 
ence is increased by the use of new terms based upon the degree of bud pro- 
tection. As a matter of fact, Raunkiaer 's system, like the others discussed 
here, takes account of both summer and winter conditions, and its difference 
is more a matter of arrangement and terminology than of essentials. For 
example, the group of phanerophytes corresponds essentially to woody plants, 
cryptophytes constitute the bulk of pleiocyclic herbs, and therophytes are 
annuals, while the subdivisions practically all have their equivalents in the 
other systems. The hemicryptophytes are far from satisfactory as a group, 
because of their similarity to helophytes on the one hand (p. 420) and thero- 
phytes on the other (p. 423). By the omission of cryptogams, the classifica- 
tion avoids confusion with systematic types and presents an attractively con- 
sistent character, increased by a consistent terminology. "While the terms are 



LIFE-FORMS 265 

well-chosen and properly constructed, their length will preclude their com- 
mon use, except perhaps in the case of the five major groups : 

I. Phanerophytes (bud-shoots aerial) : 

1. Herbaceous phanerophytes. 

2. Evergreen megaphanerophytes (above 30 m.) without bud-scales. 

3. Evergreen mesophanerophytes (8 to 30 m.) without bud-scales. 

4. Evergreen microphanerophytes (2 to 8 m.) without bud-scales. 

5. Evergreen nanophanerophytes (below 2 m.) without bud-scales. 

6. Epiphytic phanerophytes. 

7. Evergreen megaphanerophytes with bud-scales. 

8. Evergreen mesophanerophytes with bud-scales. 

9. Evergreen microphanerophytes with bud-scales. 

10. Evergreen nanophanerophytes with bud-scales. 

11. Phanerophytes with succulent stem. 

12. DeciduouB megaphanerophytes with bud-scales. 

13. Deciduous mesophanerophytes with bud-scales. 

14. Deciduous microphanerophytes with bud-scales. 

15. Deciduous nanophanerophytes with bud-scales. 

II. Chamaephytes (bud-shoots protected by snow or fallen leaves) : 

16. Suffrutescent chamaephytes : many Labiatae. 

17. Passive decumbent chamaephytes: species of Sedum, 8axifraga. 

18. Active chamaephytes: Linnaea, Empetrum. 

19. Cushion plants: Azorella, Eaoulia. 

III. Hemicryptophytes (bud-shoots at the soil level) : 

20. Protohemicryptophytes. 

A. Plants without creeping offshoots: Linaria, Verbena, Medicago. 

B. Plants with creeping offshoots, stolons, or rhizomes : Urtica, Saponaria. 

21. Subrosette plants. 

A. Plants without creeping offshoots : Caltha, Geum. 

B. Plants with creeping offshoots: Ranunculus reptans. 

22. Rosette plants. 

A. Plants without offshoots: Primula, Taraxacum, Carex. 

B. Plants with offshoots: Hieracium, Petasites. 
Plants with monopodial rosette. 

I. Monopodium with leaves but no scales. 

A. Aerial leaf and flower shoots : Trif olium pratense. 

B. Aerial shoots flower-bearing only. 

a. Without creeping offshoots : Plantago major. 

b. With creeping offshoots: Fragaria, Trif olium repens. 

II. Monopodium with both leaves and scales. 

A. Without creeping offshoots : Anemone hepatica. 

B. With creeping offshoots: Convallaria majalis. 

III. Monopodium with scales alone: Sedum rhodiola. 

IV. Cryptophytes (bud-shoots buried in the soil) : 
Geophytes : 

23. Rhizome geophytes: Polygonatum. 

24. Tuber geophytes: Cyclamen. 

25. Tuberous root geophytes : Orchis. 

26. Bulb geophytes: Allium, Lilium. 

27. Root-bud geophytes: Cirsium arvense, Monesea. 

28. Helophytes: Typha, Scivpus, Eqnisetuni, Sagittaria. 

29. Hydrophytes: Nymphaea, Zostera, Hippuris, Potamogeton. 

V. Therophytes (3); annuals: Galium apaxine, Thlaspi arvense. 



266 BASES AND CRITERIA 

Warming, 1908. — Warming (1909 : 5) has based his outline of growth- 
forms upon the following principles: 

"Just as species are the units in systematic botany, so are growth-forms the 
units in oecological botany. It is therefore of some practical importance to 
test the possibility of founding and naming a limited number of growth-forms 
upon true oecological principles. It can not be sufficiently insisted that the 
greatest advance not only in biology in its wider sense, but also in oecological 
phytogeography, will be the oecological interpretation of the various growth- 
forms : From this ultimate goal we are yet far distant. 

"It is an intricate task to arrange the growth-forms of plants in a genetic 
system, because they exhibit an overwhelming diversity of forms and are 
connected by the most gradual intermediate stages, also because it is difficult 
to discover guiding principles that are really natural. Nor is it an easy task 
to find short and appropriate names for the different types. Genetic rela- 
tionships and purely morphological or anatomical characters such as the vena- 
tion and shape of leaves, the order of succession of shoots, monopodial and 
sympodial branching, are of very slight oecological or of no physiognomic 
significance. Oecological and physiological features, particularly the adapta- 
tion of the nutritive organs in form, structure, and biology, to climate and sub- 
stratum, or medium, are of paramount importance. Cases are not wanting, 
however, in which oecological grouping runs parallel with systematic classi- 
fication. 

"In the case of the polycarpic plants it is necessary to consider, first, their 
adaptation to climate, and in particular the season unfavorable to plant life; 
secondly, the vegetative season; and finally the conditions prevailing in re- 
gard to the soil, which Schimper terms edaphic conditions. Of greatest im- 
portance is — 

"1. Duration of the vegetative shoot: Lignified axes of trees, shrubs, and 
under-shrubs ; perennial herbaceous shoots ; herbaceous shoots deciduous after 
a short period. 

"And closely associated with this is — 

"2. Length and direction of the internodes: Whether the shoots have short 
internodes (rosette-shoots) or long internodes, and whether the latter are 
erect (orthotropous) or prostrate and creeping (plagiotropous). 

"3. Position of the renewal buds during the unfavorable season high up in 
the air, near the soil, under the surface of the soil, or buried in the soil 
(geophilous). 

"Of less importance is — 

"4. Structure of the renewal-buds or of buds in general. 

"5. Size of the plant is of some moment, not only because in the struggle 
for existence the taller plants are enabled, to establish a supremacy more 
easily, but also because they are more exposed to the inclemency of climate; 
shrubs reach greater altitudes and latitudes than trees, while dwarf shrubs 
and herbs extend even further than shrubs. 

"7. The adaptation of the assimilatory shoot to the conditions of transpira- 
tion. 

"8. The capacity for social life is of great importance in the struggle be- 
tween species and consequently in the composition and physiognomy of the 
plant-community. This capacity is due in some cases to the prolific production 
of seed, but usually to more vigorous vegetative multiplication by means 
of traveling shoots, or shoots given off from the root. And this latter is to 
some extent determined by the soil (moist or wet soil, loose sandy soil, 
etc.)." 



LIFE-FORMS 



267 



Warming divides growth-forms into six classes and subdivides these into 
subclasses and types as follows: 



(2) 
a. 
b. 
c. 



(3) 
a. 



(2) 

(3) 



1. Heterotrophic growth-forms: holopara- 

sitea and holosaprophytes. 

2. Aquatic growth-forms. 

3. Muscoid growth-forms. 

4. Lichenoid growth-forms. 

5. Lianoid growth-forms. 

6. All other autonomous land-plants. 

I. Monocarpic herbs. 

(a). Aestival annual plants. 
(b). Hibernal annual plants, 
(c). Biennial-perennial herbs. 
II. Polycarpie plants, 
(a). Eenascent herbs. 

(1) Herbs with multicipital rhi- 
zomes: Silene inflata. 
Mat-geophytes. 
With stem-tubers: Crocus. 
With root-tubers: Ophrydeae. 
With bulbs: Liliaeeae. 
d. With perennial tuberous stem : 
Cyclamen. 
Rhizome-geophytes. 
On loose soil of dunes: Ammo- 
phila, Carex. 

b. On loose humus soil in forests : 

Polygonatum, Anemone 
nemorosa. 

c. On mud in water or swamp: 

Phragmites, Hippuris. 
(b). Rosette-plants. 

(1) Leaves sessile, elongated: 

Plantago, Taraxacum. 

(2) Leaves long-stalked, broad: 

Anemone, Hepatica. 

(3) Leaves succulent: Crassulaceae. 

(4) With runners: Fragaria, Po- 

tentilla auserina. 

(5) Flowers on leafy shoots: Al- 

chemilla, Geum. 

(6) Flowers on leafless shoots: 

Primula. 

Drude, 1913. — In broadening his earlier classification into a universal 
system of life- forms, Drude (1913:29) has applied the following criteria: 

1. The basic form (tree, shrub, annual or perennial herb), by the organization of 

which for a long period of years, or for a single season of growth, each plant 
maintains its own place. The method of propagation is an essential part of this 
basic form. 

2. The form and duration of the leaves. 

3. The protective devices of leaf- and flower-shoots during the period of rest. 

4. Position and structure of the organs of absorption. 

5. Flowering and fruiting in relation to reproduction as a single or recurrent process. 

On this basis, Drude makes three great divisions in which he recognizes 
55 types and many subtypes. 



6. All other autonomous land-plants — cont. 
II. Polycarpie plants — continued. 

(b) Rossette-plants — continued. 

(7) Grass-rosettes: grasses, sedges, 

Eriocaulaceae. 

(8) Musa-form: gigantic tropical 

herbs (banana). 

(9) Tuft-trees. 

1. Trunks without secondary 
growth; leaves large and di- 
vided: tree-ferns, palms, cycads. 

2. Trunks with secondary growth ; 
leaves undivided, linear; Yucca, 
Dracaena. 

3. Strelitzia-form. 

(c) Creeping plants. 

(1) Herbs: Lycopodium clavatum, 

Menyanthes. 
Dwarf shrubs : Arctostaphylus 

uva-ursi, Linnaea. 
Jungermannia-form. 

(d) Land-plants with long, erect, long- 
lived shoots. 

(1) Cushion-plants: Silene acaulis, 
Azorella. 

(2) TTndershrubs : 

1. Labiate type: Salvia, Thymus, 
Artemisia. 

2. Acanthus type: Acanthaceae. 

3. Rhizome-undershrubs : Vacci- 
nium myrtillus. 

4. Cane-undershrubs : R u b u s 
idaeus. 

5. Soft-stemmed plants : Araceae. 

6. Cactus -form: Cactaceae, Sta- 
pelia. 

7. Woody plants with long-lived 
lignified stems, canopy-trees, 
shrubs, dwarf shrubs. 



268 



BASES AND CRITERIA 



I. Aeropbytes (woody plants, perennial 
and annual herbs). 

1. Monocotyl tuft-trees :Sabal, Yucca. 

2. Monocotyl palm sbrubs and lianes : 

Bactris, Calamus. 

3. Dwarf palms: Nipa. 

4. Tree-ferns and cycads: Cyathea, 

Cycas. 

5. Needle-leaved woody plants. 

6. Dicotyl trees. 

7. Dicotyl shrubs and bushes. 

8. Dicotyl woody lianes. 

9. Mangrove-form. 

10. Lobelia-form. 

11. Tree-grasses: Bambusa. 

12. Smilaceous bushes and lianes: 

Smilax, Buscus. 

13. Leafless dicotyl rushwood and 

thorn bushes: Casuarina, Ephe- 
dra, Spartium. 

14. Few-leaved columnar woody plants : 

Adenium, Tumboa. 

15. Stemmed evergreen rosette succu- 

lents: Agave, Sempervivum. 

16. Dicotyl stem succulents : Cactaceae. 

17. Dicotyl dwarf shrubs: Calluna, 

Artemisia, Dryas. 

18. Woody parasites: Loranthus. 

19. Monocotyl giant herbs: Musa, 

Bromelia. 

20. Monocotyl root-climbers : Monstera. 

21. Eosette ferns and cycads: Aspid- 

ium. 

22. Tuber-stemmed epiphytes: Bulbo- 

phyllum, Myrmecodia. 

23. Perennial and renascent grasses: 

Andropogon, Poa, Carex. 

24. Sedges and rushes with suppressed 

leaves : Juncus, Scirpus. 

25. Erect half -shrubs: Ruta. 

26. Half-shrubs with creeping stems 

or offshoots : Linnaea. 

27. Dicotyl cushion-plants: Baoulia, 

Silene acaulis. 

28. Succulent cushion-plants: Aloe, 

Mesembryanthemum. 

29. Biennial and perennial rosettes: 

Pulsatilla, Verbascum. 

30. Renascent and annual climbers: 

Dioseorea, Ipomoea. 

31. Renascent multicipital herbs: Peu- 

eedanum, Galium. 

32. Geophilous rootstock plants: Iris, 

Circaea, Equisetum. 



I. Aerophytes (woody plants, perennial 
and annual herbs) — continued. 

33. Geophilous tuber plants: OrchiB, 

Cyclamen. 

34. Geophilous bulb plants: Allium, 

Oxalis. 

35. Monocotyl therophytes : Eragrostis. 

36. Dicotyl therophytes : Chenopodium. 

37. Dicotyl short-lived herbs : Koenigia. 

38. Saprophytic and parasitic herbs: 

Corallorhiza, Monotropa, Cuscuta. 
II. Water plants: 

39. Amphibious slime-rooted plants 

with aerial leaves: Sagittaria, 
Nelumbo, Marsilea, Equisetum. 

40. Amphibious free-swimming plants 

with aerial leaves: Pistia, Eich- 
hornia. 

41. Amphibious plants rooting on 

stones : Podostemaceae. 

42. Hydrophytes with rooting axis 

and immersed leaves: Isoetes, 
Zostera, Lobelia. 

43. Hydrophytes with rooting axis 

and floating leaves: Potamoge- 
ton, Nymphaea. 

44. Free-swimming hydrophytes: 

Lemma, Utricularia, Azolla. 
III. Life forms of mosses and thallophytes : 

A. Aerophytes: 

45. Terrestrial cushion-mosses: Leuco- 

bryum. 

46. Terrestrial tall-stemmed mosses: 

Polytrichum. 

47. Terrestrial and epiphytic mat- 

mosses : Hypnum, Frullania. 

48a. Petrophilous creeping mosses, 
chiefly liverworts : Marchantia, 
Jungerm annia . 

48b. Petrophilous mat- and cushion- 
mosses: Georgia, Andreaea. 

B. Hygrophytes and hydrophytes: 
49. Bog mosses : Sphagnum. 

50a. Streaming mosses: Fontinalis. 
505. Forming mats in water: Aneura, 
Scapania. 

51. Epiphytic lichens: Usnea. 

52. Fruticose and foliose lichens on 

rocks and earth: Cetraria, Um- 
bilicaria, Cladonia. 

53. Crustose lichens: Lecanora. 

54. Forms of marine algae, green 

algae, bluegreen algae, etc. 

55. Forms of saprophytic and para- 

sitic fungi. 



Comparison of the systems. — The three systems of Raunkiaer, Warming, 
and Drude differ greatly as to the manner of classification, but they are in 
much greater harmony as to the essential basis. Drude, however, constantly 



VEGETATION-FORMS 269 

uses taxonomic criteria, though he is very far indeed from consistent, sepa- 
rating monocotyls, dictoyls, and ferns sometimes into distinct types, some- 
times into subtypes, and then frequently uniting two of them or all three into 
the same type or subtype. Raunkiaer ignores taxonomy altogether and 
Warming practically does the same, with the exception of the thallophytic 
forms, in which taxonomic form and life-form are more or less identical. The 
treatment of aquatics, in which the impress of the habitat is marked, is very 
different in the three cases. Raunkiaer makes helophytes and hydrophytes 
two types of cryptophytes, coordinate with geophytes. Warming treats 
aquatic plants as one of his six main divisions, though he considers them 
under ecological classes or habitat- forms (136), while Drude makes water 
plants one of his two great divisions of flowering plants and recognizes three 
amphibious and three aquatic types. Raunkiaer uses bud-position as the 
primary criterion for his five main groups (all flowering plants and ferns). 
Warming employs systematic criteria for two of his six divisions, ecologic for 
three, and physiologic for one. Land-plants are divided upon the nature of 
the life-period into monocarpic and polycarpic. Drude 's first division is 
ecologic for aerophytes, and water-plants, and systematic for mosses and 
thallophytes. In all three systems the types and subtypes are frequently the 
same, except that Drude usually divides the same type or subtype upon the 
basis of taxonomy. 

The systems of Raunkiaer and Drude are the most unlike, while Warming's 
occupies an intermediate position. Raunkiaer 's classification is much the 
most compact and consistent, probably because he has adhered to one cri- 
terion throughout. Because of this, and because he has given definite names 
to practically every type, it is also much more usable. In fact, its great merit 
lies in the possibility of using it as a sort of climatic index, while the other 
two systems merely classify a great mass of plants in the usual static fashion. 
As Warming points out, Raunkiaer 's system has one disadvantage in that it 
fails to take account of the growing season response (1906:6) and hence 
applies to the flora and not to the vegetation of a region or country. 

Vegetation -forms. — For our purpose, much the most useful and consistent 
view of life-forms is obtained from a single point of view, that of vegetation. 
The development and structure of vegetation are chiefly a matter of domi- 
nants and subdominants, and it is the life-forms shown by these which are of 
paramount importance. Hence it becomes desirable to speak of them as 
vegetation-forms, as Drude did originally, following Grisebach and Humboldt. 
For practical purposes, it is undesirable to make a complete classification of 
vegetation-forms and the latter is carried only so far as the demands of indi- 
cator vegetation warrant. 

The dominance of a species depends upon the perfection of its methods of 
increase on the one hand, and upon the success of its vegetative shoots in 
competition on the other. While the latter is partly a matter of length of 
shoot and rate of growth, it is chiefly one of carrying the shoots of one season 
over to the next. A wholly consistent and usable system is possible upon the 
basis of these three processes. It avoids the complexities and uncertain cor- 
relations introduced by taxonomy and permits a consistent treatment of 
habitat-forms with their more evident factor correlations. It contains the 



270 BASES AND CRITERIA 

essentials of the systems discussed above, inasmuch as Drude states that 
the basic life-forms are trees, shrubs, perennial and annual herbs, Warming 
divides his group of land-plants into monocarpic and polycarpic, while 
Eaunkiaer's largest groups, phanerophytes, cryptophytes, and therophytes, 
practically correspond to woody plants, perennial and annual herbs. In 
giving more or less equal value to the life-period, method of over-wintering, 
and conservation of shoots and success in competition, it appears desirable to 
recognize four coordinate groups, viz., annuals, biennials, herbaceous peren- 
nials, and woody perennials, characterized as follows: 

1. Annuals: Passing the winter or dry season in seed or spore form alone; no propaga- 

tion or accumulation of aerial shoots; living one year. 

2. Biennials: Passing one unfavorable season in the seed or spore form, and the next as a 

propagule; no accumulation of aerial shoots; living two or parts of two years. 

3. Herbaceous perennials : Passing each unfavorable season in both seed or spore and 

propagule form; no accumulation of aerial shoots; living several to many years. 

4. Woody perennials: Passing each unfavorable season as seeds or spores, and aerial 

shoots or masses, often with propagule forms also, especially when injured; living 
many seasons as a rule. 

Each of these divisions is thoroughgoing and all forms of annual habit are 
placed in the first group, whether flowering plants, mosses, or fungi, just as 
perennials are placed in their respective group regardless of their systematic 
position or habitat-form. The varying nature of the four groups makes it 
obviously impossible to employ the same criterion for the division into types. 
For annuals and biennials, the form of the aerial plant body is probably of 
first importance and the size next, while for woody plants height is perhaps 
most decisive, leaf-character next, and form last. While perennial herbs 
usually show the most marked differences in the propagules, the form of the 
aerial shoot is often even more distinctive, and both criteria must be employed 
as occasion warrants. The final result is a simple compact system, closely 
resembling the earlier one of Drude (1896; Pound and Clements, 1900) 
and different but little in essence from that of Eaunkiaer. For the study 
of indicators only the major divisions appear to be of value at present, and 
these alone are given in the outline. 

1. Annuals. 6. Cushion-herbs. Woody perennials. 

2. Biennials. 7. Mat-herbs. 11. Halfshrubs. 
Herbaceous perennials: 8. Bosette-herbs. 12. Bushes. 

3. Sod-grasses. 9. Carpet-herbs. 13. Succulents 

4. Bunch-grasses. 10. Succulents. 14. Shrubs. 

5. Bush-herbs. 15. Trees. 

Indicator significance of vegetation-forms. — It is obvious that the vegeta- 
tion-forms of climax dominants are indicators of climate. This has long been 
recognized as the basis for the climatic zones of continents and mountains. 
The same principle applies to climax formations generally; and these are 
accordingly taken as indicators of the major climates of the globe (Clements, 
1916). This close correlation between the major vegetation-forms and climate 
as expressed in progressively favorable conditions of temperature and mois- 
ture is paralleled by the succession of vegetation-forms in the development 
of a climax. In the development of a sere, extreme conditions as to water 
yield to those more and more favorable to growth, and this change is accom- 
panied by a sequence of dominants belonging to successively higher vegeta- 



HABITAT-FORMS 271 

tion-forms. In short, the more striking indicator values of succession are 
afforded by the changes from one vegetation-form to another, just as those 
next in importance are marked by the change from one associes to another of 
the same form. Moreover, while the exact significance of any species can be 
known only by determining its functional response to the factors of its habi- 
tat, its general meaning is indicated by the vegetation-form to which it 
belongs. 

Eaunkiaer (1905, 1908; Smith, 1913:16) has employed his system of vege- 
tation-forms to determine the climatic relations of a particular flora. He 
establishes a hypothetical normal spectrum for the whole earth by selecting 
1,000 representative species, of which 400 were carefully analyzed. The bio- 
logical or phy to-climatic spectrum of a particular region is obtained by find- 
ing the percentage of species belonging to each life-form. Eaunkiaer 's method 
adds interest and detail to the long-accepted relations between climate and 
flora. It can not be applied to vegetation and hence it has no real indicator 
value, as is shown by the author's own statements (1905 :433) : 

"If we consider the flora of Denmark, it is characterized from the botano- 
climatic viewpoint by its hemicryptophytes and not by its phanerophytes, for, 
however important may be the role played by the forests in the vegetation of 
Denmark, the small number of species of phanerophytes is significant of the 
conditions offered by this region : The species of phanerophytes represent but 
6 to 7 per cent of those living in Denmark, while the hemicryptophytes con- 
stitute nearly a half of all the species. 

"But from the standpoint of the formation, the phanerophytes, or trees, 
dominate by their size wherever one finds them. In spite of the inferiority in 
number of the species of phanerophytes to those of hemicryptophytes or 
cryptophytes, our forests belong to the phanerophytic formations because the 
phanerophytes they contain dominate the other components of the forests." 

HABITAT-FORMS. 

Concept and history. — In addition to the taxonomic form and vegetation- 
form, species exhibit a form which is much more distinctly related to the 
habitat. These usually bear the clear impress of the latter and hence are 
called habitat-forms. The fuller recognition of their basic importance by 
Warming (1895, 1896:116) was largely responsible for the rapid develop- 
ment of ecology during the last two decades. Unlike taxonomic forms and 
vegetation-forms, their value is primarily ecological and not floristic, and they 
are of correspondingly greater importance as indicators. Their significance 
lies in the fact that they bear the primary impress of the controlling or lim- 
iting factor, and thus serve as direct indicators of the critical factors of the 
habitat. They are the essential basis of all indicator values, and must be 
regarded as the main objective in all such studies. 

Warming's system. — Warming (1896 : 116) was the first to adequately 
organize the four universally known groups of habitat-forms, namely, hydro- 
phytes, xerophytes, halophytes, and mesophytes (cf. Clements, 1904:20). 
Pound and Clements (1898:94; 1900:169), feeling the need of recognizing 
light as well as water, divided mesophytes primarily upon the basis of light 
and combined halophytes with xerophytes, thus establishing the following 
six groups: hydrophytes, mesophytes, hylophytes, poophytes, aletophytes, 
and xerophytes. This division of mesophytes retained some idea of life-forms, 



272 BASES AND CRITERIA 

and it was later dropped (1902:166; 1907:183) for the consistent light 
grouping of mesophytes into heliophyta, sciophyta, and scoiophyta, corre- 
sponding essentially to Schouw's classification into sun, shade, and darkness 
plants (1823:166). A detailed classification of habitat-forms was made by 
Clements (1902:5-14), in which light, solutes, aeration, and other factors 
were taken into account, but with water-content as the primary basis. The 64 
subdivisions were largely successional and physiographic, and this number 
can be greatly reduced if factors alone are considered. This is essentially 
what Warming has done in his most recent grouping of formations (1909: 
136), which also represents much the best classification of habitat- forms up 
to the present. This system is as follows : 

A. The soil (in the widest sense) is very wet, and the abundant water is available 

to the plant; the formations are therefore more or less hydrophilous: 
Class 1. Hydrophytes (of formations in water). 
Class 2. Helophytes (of formations in marsh). 

B. The soil is physiologically dry, i. e., contains water which is available to the plant 

only to a slight extent; the formations are therefore composed essentially of 

xerophilous species : 
Class 3. Oxylophytes (of formations on sour (acid) soil). 
Class 4. Psychrophytes (of formations on cold soil). 
Class 5. Halophytes (of formations on saline soil). 

C. The soil is physically dry, and its slight power of retaining water determines the 

vegetation, the climate being of secondary import ; the formations are therefore 

likewise xerophilous: 
Class 6. Lithophytes (of formations on rocks). 
Class 7. Psammophytes (of formations on sand and gravel). 
Class 8. Chersophytes (of formations on waste land). 

D. The climate is very dry and decides the character of the vegetation; the proper- 

ties of the soil are dominated by climate; the formations are also xerophilous: 
Class 9. Eremophytes (of formations on desert and steppe). 
Class 10. Psilophytes (of formations on savannah). 
Class 11. Sclerophyllous formations (bush and forest). 

E. The soil is physiologically or physically dry : 
Class 12. Coniferous formations (forest). 

F. Soil and climate favor the development of mesophilous formations: 
Class 13. Mesophytes. 

Modifications of Warming's system. — In making use of habitat-forms as 
indicators in North American vegetation, a few modifications of the above 
groups are desirable. These are perhaps further warranted by some advance 
in ecological knowledge in the ten years since Warming made the following 
statement concerning habitat-forms (1909:133): 

"When endeavoring to arrange all land-plants, omitting marsh-plants, into 
comprehensive groups, we meet with first some communities that are evidently 
influenced in the main by the physical and chemical characters of the soil 
which determine the amount of water therein ; secondly, other communities 
in which extreme climatic conditions and fluctuations, seasonal distribution 
of rain and the like, decide the amount of water in soil and character of 
vegetation. In accordance with these facts, land-plants may be ranged into 
groups, though in a very uncertain manner. The prevailing vagueness in this 
grouping is due to the fact that oecology is only in its infancy, and that very 
few detailed investigations of plant-communities have been conducted, the 
published descriptions of vegetation being nearly always^ one-sided and 



ECADS 273 

floristic, as well as very incomplete and unsatisfactory from an oecological 
standpoint. ' ' 

The terms employed are those suggested by Clements (1902:5) and adopted 
by Warming for most of his divisions : 

I. Hydrophytes: Chresard maximum to very high, the soil being water or covered with 
water; climate usually moist. 

1. Emophytes: Entire plant submerged; no transpiration or functional stomata. 

2. Plotophytes: Plant floating, at least the leaves; transpiration and stomata on 

upper surface of leaves at least. 

3. Helophytes: Amphibious, rooted in water or mud; transpiration high and 

stomata on both surfaces, the stem often functioning as a leaf. 

II. Mesophytes: Chresard medium, soil moist; climate moist; transpiration high to 
medium. 

4. Heliophytes : Sun-plants, growing in sunlight or light stronger than 0.10. 

5. Sciophytes: Shade-plants, growing in light less than 0.10. 

III. Xerophytes: Chresard low, soil physically or physiologically dry, climate usually dry, 
or various ; transpiration low. 

A. Soil physiologically dry, climate various: 

6. Halophytes: Chresard low, due to an excess of soil salts. 

7. Psychrophytes : Chresard low, due to cold soil or to ice. 

8. Oxyphytes: Chresard low, due to lack of oxygen in the soil. 

B. Soil physically dry, climate various: 

9. Lithophytes: Chresard low, due to a rock matrix. 

10. Psammophytes : Chresard low, due to sandy or gravelly soil. 

11. Chersophytes : Chresard low, due to a rock substratum. 

C. Climate dry and soil physically dry in consequence: 

12. Eremophytes : desert plants, chresard low or lacking much of the year. 

13. Psilophytes : grassland plants (prairie, plains, steppes), chresard low some of 

the year. 

14. Drymophytes: bushes, shrubs, and small trees, mostly sclerophyll scrub, chap- 

arral, and woodland; chresard low or discontinuous. 

The changes from Warming's system lie in the subdivision of hydrophytes 
and mesophytes, well-recognized distinctions of which Warming himself makes 
use (18, 165), in the distribution of conifers among helophytes, mesophytes, 
psammophytes, and drymophytes, in the line drawn between desert and 
grassland plants, and in treating the bush-shrub form as primary and the 
division into sclerophyll and deciduous types as secondary. 

Indicator value. — Habitat-forms are the most satisfactory of all indicator- 
forms. This is chiefly because of their obvious response to the controlling 
factors which the forester, grazing expert, and others must deal with. This is 
partly also because they mark out a definite area in which these factors pre- 
vail. For all practical purposes in a particular region, habitat-forms con- 
stitute the ground-work of an indicator system. This is evident when it is 
realized that the fourteen groups comprise all dominants and thus each 
habitat-form has a community value as well. When reinforced by vegetation- 
forms in so far as their significance for climate is known, and by ecads and 
growth-forms for the more recent or the minor effects of physical factors, 
habitat-forms afford a nearly complete system of indicators for the practical 
application of biology. It is still necessary to interpret some of them with 
greater accuracy and certainty. This will come about from the quantitative 
study of their physiologic response, permitting the closer correlation of form 

19 



274 BASES AND CRITERIA 

and function, as well as by the increasing use of standard plants as even 
more accurate indicators. 

Habitat-forms can be used to give a general statistical expression to the 
climatic or physiographic conditions of a region, and thus permit comparisons, 
much as Kaunkiaer has used vegetation-forms. Their paramount value lies 
in their positive indication of definite local conditions on the basis of known 
correlation with measured factors. It should be noted that the mesophytes 
and the last three groups of xerophytes represent climax habitats and conv- 
munities, while the hydrophytes and the first six groups of xerophytes char- 
acterize developmental stages. This is a natural outcome of the fact that the 
climate is controlling as to soil conditions in the former, while the climatic 
control is much reduced or is none at all for the latter. The general correla- 
tion of climax habitat-forms and their most important representatives with 
physical factors is given in "Plant Indicators" (p. 105), in so far as quanti- 
tative results are available. 

In a recent paper, Eaunkiaer (1916:225; cf. Fuller and Bakke, 1918:25) 
has sought to express the general relation of plants to climate by a series of 
leaf classes based upon size. Of the latter, he recognizes six kinds as follows : 
leptophyll, 25 sq. mm. ; nanophyll, 9 X 25 sq. mm. ; microphyll, 9 2 X 25 
sq. mm. ; mesophyll, 9 3 X 25 sq. mm. ; macrophyll, 9* X 25 sq. mm. ; megaphyll. 
While this classification will serve a useful purpose in drawing the attention 
of ecologists to such relations, it seems quite too subjective for final accep- 
tance. This seems obvious from the author's difficulties as to compound and 
lobed leaves, and especially from the following statement (1. c, 29) : 

"Originally I multiplied by 10, but the resulting limits between the 'size- 
classes' did not seem as natural as when 9 was used. It is easy in the final 
analyses to separate the single classes into the groups of small, medium, and 
large. ' ' 

Thus, while there can be little question that leaf-size often serves as an 
indicator of climate or habitat in some degree, it must be refined by means of 
leaf-number, thickness, structure, outline, and texture, and checked by quan- 
titative studies of factors (cf. E. S. Clements, 1905:91). 

Ecads. — An ecad is produced by direct and demonstrable adaptation to 
a habitat. It is a habitat-form in the making. The habitat-form, while 
capable of modification within certain limits, has recorded the impress of a 
particular habitat for so long that its general character is fixed and trans- 
mitted. An ecad, though it may show just as striking adaptation, is a recent 
product, and its character is not yet fixed and transmissible. The difference 
between the two is solely one of inheritance, and it seems probable that ecads 
become fixed and pass over into habitat-forms after a long residence in the 
same habitat. This is indicated by the behavior of alpine dwarfs, some of 
which retain their form when moved to lower altitudes or shifted to wetter 
alpine situations, while others at once change in response to the new condi- 
tions. The former have attained the stability of habitat-forms, the latter are 
ecads. 

Because of its plastic nature, the ecad is a more exact and sensitive indi- 
cator than the habitat-form. Its structural change corresponds more nearly 
to the functional response and can be regarded as a measure of the latter to a 



GROWTH-FORMS 275 

considerable degree. Its growth as well as its form is often characteristic, and 
its indicator value can be based upon both. One unique advantage of the 
ecad is that it is produced in abundance in nature, wherever habitats touch, 
especially where they recur constantly, as in mountain regions. A plastic 
species found in two or more habitats regularly shows an ecad corresponding 
to each. Similar results are readily obtained by transplanting such species 
to several different habitats. Ecads produced under definite quantities of 
water and light may be grown under control (Clements, 1905:157; 1919) 
and used for comparison with the natural ones (E. S. Clements, 1905) 
(plate 11). 

Ecads have been classified and named with reference to habitats, as hylo- 
colus, psilocolus, etc. (Clements, 1902:17; 1904:329). It seems much better 
to group and designate them with reference to the controlling factor (Clem- 
ents, 1908:263), as water ecads, light ecads, etc. Thus the general classifica- 
tion of ecads would necessarily correspond closely to that of habitat-forms, 
except in xerophytes, where the groups would be fewer. Such a classification 
would be of little value, however, since it is the relationship of the ecad to a 
particular species which is significant, as well as the number and kind of 
ecads actually occurring. A floating species, such as Sparganium angusti- 
foUum, forms both submerged and amphibious ecads, while Nymphaea polyse- 
pala has been seen to produce only amphibious ones. A plastic helophyte, such 
as Ranunculus sceleratus, or a mesophyte, such as Achillea millefolium, may 
give rise to several ecads. The same species may produce both water and 
light ecads, though as a rule a wide range of adaptation to the one factor 
is accompanied by a narrow range for the other. Under control it has been 
possible to produce ten distinct water ecads of Ranunculus, but beyond this 
point differences have to do chiefly with amount of growth rather than with 
structure. For the present, it is sufficient to recognize the controlling factor 
by designating ecads as hydrads, xerads, sciads, heliads, halads, etc., and to 
leave the question of a more exact terminology for the future. The impor- 
tance of ecads in indicator work is so great that their recognition can no 
longer be neglected. 

GROWTH-FORMS. 

Nature. — While it is assumed that all plant forms are referable to the 
immediate or remote action of the habitat, this correlation is least certain for 
taxonomic forms. Its certainty increases progressively through life-forms 
and habitat-forms to reach a maximum in growth-forms. While Warming in 
particular has used this term in place of life-form and vegetation-form, the 
latter have the preference, both bj r priority and significance. But growth- 
form is such a desirable term for the immediate quantitative response made 
by a plant to different habitats or conditions that its retention in this sense 
seems well-warranted. As the direct visible response of the plant, to physical 
factors, growth affords a more delicate scale of measurements even than the 
ecad. In fact, the latter is only a growth-form in which adaptation as shown 
by a qualitative change of form or structure is more striking than the quanti- 
tative difference in amount of growth. In the case of dwarfing, both changes 
usually occur together, and the growth-form differs from the ecad only in 
being the product of the conditions presented by a single season. If these 



276 BASES AND CRITERIA 

continue, the growth-form persists and becomes an ecad characteristic of the 
particular habitat. Thus, while the two forms may be measures of the same 
conditions, the one is an indicator of the annual variation, the other of the 
normal condition of the habitat. From the ecological side, it appears that 
growth-forms may become ecads, ecads become habitat-forms, and these finally 
fixed as vegetation-forms. 

Kinds. — Every direct factor exerts an influence upon growth and pro- 
duces corresponding growth-forms. Such factors are water, light, tempera- 
ture, and aeration, and possibly certain solutes. Since all of these are con- 
cerned in the growth of each plant, it is possible to assign a particular one as 
the cause of any growth-form only when it is the controlling or limiting 
factor. In the majority of cases, the limiting action is evident, as with water 
in arid and semi-arid habitats or dry seasons, light in forests and thicket, 
temperature in high altitudes or latitudes or cold seasons, and aeration in wet 
areas or seasons. Maximum growth results when all four factors are at the 
optimum for a particular species. An apparent exception is afforded by the 
behavior of many species in moderate shade, but their height is usually offset 
by their slenderness, and the mass growth and dry weight are usually less 
than in the sun. With the optimum growth as the basis, it becomes possible 
to distinguish growth-forms due to the extremes of each factor, as well as to 
correlate different amounts of growth with known quantities of the limiting 
factor. In the case of water, growth is decreased by both an excess and 
deficit as a rule, but the former seems to operate through reduced aeration 
and lowered temperature. Similarly, growth is diminished by both high and 
low temperatures, but high temperatures act chiefly through the water rela- 
tion. It is doubtful whether full sunshine as light ever inhibits growth, since 
photosynthetic activity decreases with any material reduction in light intens- 
ity. While many species are taller and more branched in moderate shade, it 
appears that mass growth is at a minimum and often becomes completely 
impossible with the increasing density of forest or thicket. 

As a consequence of the above, it is most practical to distinguish four types 
of growth-forms, based upon the lack of the direct limiting factors, namely, 
those due to insufficient water, to insufficient heat, to shade, and to poor 
aeration. Since growth is primarily quantitative, each species will exhibit a 
series of forms from the optimum to the minimum, corresponding to each 
effective degree of change in the limiting factor. This relation lies at the base 
of ecological response and can only be determined experimentally. Two 
factors may act together in producing a growth-form, as in the case of alpine 
dwarfs due to drouth and low temperature. One factor may serve to empha- 
size another, as where the drouth of a desert is reinforced by an excess of 
salts in the soil, or it may decrease or counteract the effect of another, as is 
true of shade in arid regions. Finally, all four factors may be concerned 
causally in an effect produced directly by one of them. This is apparently 
the case in the death of sal seedlings in tropical forests, as shown by Hole 
and Singh (Chapter XIII). The immediate cause is poor aeration, due to the 
accumulation of soil-water as a consequence of lower temperature resulting 
from shade. 

Indicator relations. — The growth of a species varies from one year to the 
next, and from one habitat to another. It often differs also in different por- 



GROWTH-FORMS 277 

tions of the same habitat. In an area which is uniform physically, individuals 
frequently show striking variations due to competition. These four relations 
sum up the indicator values of growth-forms as they occur in nature and 
hence serve as the basis of all correlations. While they are well-known, little 
quantitative work has yet been done with them. This has been due to the 
time necessary to organize quantitative studies and methods out-of-doors and 
to focus these upon growth as the most basic of visible responses. Pearson 
(1918) has made measurements of the annual growth in height of yellow-pine 
seedlings for a period of six years and has found a close correlation with 
spring rainfall. Sarvis (1919) has clipped and weighed the growth on per- 
manent grass quadrats at intervals of ten days and has made a general corre- 
lation with seasonal factors. Since species vary greatly in rate and amount 
of growth, it is desirable to select those most responsive to the habitat. 

It is impossible to say as yet what type of growth is most readily correlated 
with seasonal variations or habitat differences. Theoretically, it seems that 
total growth as indicated by the dry weight of mature plants would furnish 
the best correlation (cf. Pearson, 1918; Frothingham, 1919; Sarvis, 1919). 
Actually, however, vegetative growth and reproductive growth make different 
demands, and are often antagonistic to each other. This is true to a large 
degree of the height-growth and width-growth of woody plants. The determi- 
nation of dry weight is a practical impossibility for trees except when young, 
and the indicator correlation must be with growth directly. At present it 
is only possible to say that for the first 100 to 150 years height-growth offers 
the better correlation, and after this period growth in diameter reflects condi- 
tions more accurately. Mitchell (1918:23) has shown in the case of incense 
cedar (Libocedrus decurrens) that the mean height-growth for the first 100 
years was 65 feet, for the second century 28 feet, for the third 12 feet, and 
for the fourth 6 feet. The width-growth was 13 inches, 14 inches, 9 inches, 
and 5 inches for the same periods. Thus practically 60 per cent of the height- 
growth was made in the first century, and but 31 per cent of the width- 
growth, while the height-growth of the fourth century was but 5 per cent in 
contrast to a width-growth of 12 per cent. The correlation of reproductive 
growth and especially of seed-production with seasonal or habitat conditions 
is known only to the extent that it tends to rise with less favorable conditions 
as to water up to a certain point, as shown by alpine and arid regions. For 
most woody plants it is little or none in youth, and it increases steadily up 
to maturity. In the case of crop plants, it seems clear that the correlation 
with dry weight offers a satisfactory basis for comparison, though even here 
greater accuracy can be expected from the separate correlation of vegetative 
and reproductive growth with the controlling factors in the two periods. 

Standard plants for growth correlations. — Because of the control possible 
as well as the opportunity for measuring functional responses, standard plants 
offer much the best method of establishing growth correlations. The value 
of the method increases as the standard plant approaches the one to be 
indicated in character, and reaches a maximum when the latter is itself 
employed as a standard, as in the use of yellow pine, Douglas fir, etc., in 
forest investigations. The employment of phytometers in this form is the 
most basic of all quantitative methods and is destined to play the paramount 
role in all exact studies of communities and habitats in the future. 



278 BASES AND CRITERIA 

Competition-forms. — The amount of a particular factor available for any 
species or individual is either determined by the habitat alone or by com- 
petition. In the great majority of cases, the major limits are fixed by the 
habitat, and within these competition determines the amounts available for 
each plant. Indeed, this is probably true of all communities except those 
initial ones in which the individuals are widely scattered. In nearly all cases, 
then, a growth-form is due partly to the nature of the habitat and partly to 
the modification of this by competition. The part played by each can be 
determined only by actual experiment or by the comparison of individuals 
growing in the same habitat but in areas with and without competition. 
Fortunately, such areas are of sufficient frequence in nature to reveal the 
normal growth-form of the habitat as well as the growth-form due to com- 
petition. A study of the chaparral and strand communities of southern 
California (Clements and Clements, 1916) disclosed an unusually large num- 
ber of such competition-forms, especially among the annuals, as would be 
expected. While competition-forms are probably just as frequent among 
perennials, they are often much less striking. 

As competition may occur in all degrees in accordance with the number 
and density of individuals, so there may be a complete series of forms from 
the normal to the extreme in which the plant never develops beyond the 
seedling stage before it dies. Under somewhat less severe competition, plants 
develop stems and leaves but fail to form flowers and fruit. In the next 
degree, reproduction occurs, but the flowers are single or few, while beyond 
this are more and more perfectly developed forms until the optimum for the 
habitat is reached. Each form is an index to some degree of competition, but 
its exact indicator value is more difficult to determine. This is due largely to 
the fact that competition has as yet received but little attention, especially 
on the experimental side. The view advanced by Clements (1904:166; 
1905:310; 1907:251; 1916:72) that competition is purely physical seems 
to be confirmed by recent experiments. While it is perhaps unnecessary to 
rigidly exclude metaphor in connection with competition, it should be recog- 
nized that the experimental results so far obtained show that plants do not 
compete for "room." Competition has to do only with the direct factors of 
the habitat. Water and light are the factors universally concerned, though 
soil-air, nutrients, and heat must also be taken into account in particular 
habitats. In addition, there is often more or less decisive competition between 
the flowers of a community for pollination agents. Furthermore, the course 
of competition may be determined by a deleterious substance, especially a 
solute, which handicaps one species more than another. Such a handicapping 
influence is even more frequently represented by biotic agents, parasitic 
plants, rodents, grazing animals, etc. 

The competition-forms commonly met with are due to competition for 
water or light, or for both together. There has been no experimental study 
of competition for soil-air or for nutrients, and it is impossible to assert at 
present that plants do compete for heat. Studies of germination under differ- 
ent densities of seeding suggest such competition for seedlings at least. No 
adequate study of competition-forms has been made, and hence it is impos- 
sible to relate them to definite quantities of water or light. In fact, it seemB 
increasingly probable that the forms resulting from intense competition are 



COMMUNITIES AS INDICATORS 279 

due to a lack of both factors, though in different degree. As a consequence, 
competition-forms can at present be used directly only as indicators of the 
general degree of competition. In connection with the habitat-form or ecad, 
they have an indirect value in making it possible to distinguish in indicators 
the direct effect of the habitat as contrasted with the added effect of com- 
petition. 

COMMUNITIES AS INDICATORS. 

Value. — The community as an indicator is a complex of all the preceding 
values. It derives its primary significance from the dominants, chiefly 
through their life-forms and ecological requirements. It includes the mean- 
ings of the less significant subdominants, and those of the much less important 
secondary species. In short, it is a complete scale upon which all the indica- 
tions of the habitat are written. These values can be obtained only by 
analysis, however, and the latter leads at once to the study of dominants and 
subdominants, both climax and serai. The general principles of the latter 
have already been outlined under the sections on associational and succes- 
sional bases. This leaves for consideration the various types of communities 
and the functions and structures they exhibit. 

Kinds of communities. — With reference to association alone, three kinde 
of communities may be distinguished, viz., consocial, 1 associal, and mixed. 
The first consists of a single dominant, the second of two or more belonging 
to the same association or serai stage, and the third of dominants from differ- 
ent associations or associes. The basic indicator value of these is determined 
by whether they are climax or serai. The consocial community affords the 
most definite indication, while the associal type has the advantage of checking 
the indications of one dominant by those of the related ones. This is even 
truer in the case of mictia, but the indications are necessarily somewhat con- 
fused here, since one set of dominants is disappearing and the other increas- 
ing in number and importance. In this connection it is desirable to emphasize 
the fact that serai and climax communities furnish not only indications of 
existing factors and possibilities, but also of past and future ones. Each serai 
stage indicates the preceding stage and its habitat. The climax forecasts the 
consequences of any primary or secondary disturbance in it, and foreshadows 
the effects of climatic changes. As a result, both serve as invaluable indi- 
cators of the course and outcome of all possible human practices in them, 
and lend themselves to methods of scientific prophecy which can hardly be 
surpassed. A similar relation exists between consocial and associal communi- 
ties. Wherever a consocies or consociation is found, the related dominants 
have occurred or can occur, at least with the slightest modification of the 
habitat. Thus, the indicator analysis of a community involves not only the 
measurement of existing conditions, but especially also a study of the linkage 
with the other communities of the sere or the climax. For indicator research, 
as in all serious ecological studies, any investigation which fails to take full 
account of successional and climax relations is inadequate, and at best can 
only lead to half-truths. 

'This term is here used to refer to the community marked by a single dominant, whether 
consocies or consociation, and associal in a similar sense. Both terms are also used to 
refer definitely to consocies and associes respectively, but the context is usually decisive. 



280 BASES AND CRITERIA 

The basic correlations of communities may be illustrated by the following 
diagram (fig. 2) : 

Climax Formation. 

society - consociation - association - ecotone - association - consociation - society 

t. f 

socies - consocies - associes - subclimax - associes - consocies - socies 



(Prisere) associes. associes. (Subsere) 



f 

mictium. 


.t 

mictium. 


t 


t 


associes. 


associes. 


+ 


t 


associes. 


associes. 


t 


+ 


associes. 


associes. 


t 

associes. 

t 

associes. 

t 


f 
colony. 

t 
family. 


associes. 




t 

colony. 

+ 
family. 





Fig. 11. — Diagram of the climax and serai communities of the formation. 

Community structures. — In addition to the units themselves, associal and 
consocial communities show general structural features, such as zones, alternes, 
layers, and aspects. These are due primarily to the grouping or appearance 
of the subordinate communities with reference to a particular factor or factor- 
complex, and are of the greatest indicator value. The well-known zonation 
of the hydrosere in and about ponds is the best example of this. Each zone 
not only marks the general factor limits for its proper community, but also a 
distinctive step in the decrease of w